user16705: I was watching the match between Ian Nepomniachtchi vs Hikaru Nakamura and Nakamura used both hands to castle.
Why is it illegal to castle using both hands? What's wrong with that?

user-1: It's because you can't save yourself time by moving with more than one hand like that. Imagine one minutes where people did that. Castle would be over powered for time management reasons. Therefore, you castle with one hand, and the problem is avoided.

user15221: This is speculation, but I think it's because FIDE wants to reduce the difficulty of knowing whether the rules are followed. One must use the same hand to move the piece and hit the clock because FIDE wants people to not hit the clock before they finish their move, and if different hands were used for the two actions, it could be unclear which happened first. And if someone uses both of their hands for a move, then clearly at least one of the hands used in the move will be different from the one used to hit the clock, making it difficult to see whether the other hand completed its portion of the move before the clock was hit. And it's not just what is easy to see, but what is clearly distinct. If we're trying to decide which hand completed its motion first, that is a matter of perception. But if we're trying to decide whether two hands were used, that's a clear binary. It's easy for two players to have differing opinions as to which hand completed its motion first, but whether two hands were used is an objective fact, and if the players disagree, that strongly suggests that at least one of them is just straight up lying.

user17210: In chess, whenever you touch a piece, you MUST move that piece. If you were to begin castling using two hands, and then removed your hands, it would be unclear which piece you would be required to move. This is why it must be done with one hand.

user6885: It would permit a rather interesting ambiguity on the board.
Many have mentioned that the king has to move 2 spaces first, so that there's only one possible valid thing to do.  So let's assume we do this.
With one hand, move the king two spaces.  Once the king is in place (proving you intend to castle), the other hand grabs the rook and moves it.  Meanwhile, the first hand goes to hit the clock.
It is possible that the first hand hits the clock before the rook is moved.  Now you have a really funny situation where your hands are still moving pieces while it is the opponent's turn!  Needless to say, this could be a big deal, especially if the winning move is to take that rook.
To avoid this case, one needs rules which ensure that all pieces are in their final position before the clock is pressed.  Whatever rule FIDE chose, this is the key requirement placed on that rule.  Hands off the board when it's not your turn!
If we use pi caluclus we can define a process where the clock press must occur after both moves are complete, regardless of which hand does what.  So in theory, one might define a different rule which permits two-handed operation.  However, in real life practical situations, such definitions are unreasonable. They work great with transistors and wires, and less well with human hands.  Instead, we simplify the process by not allowing things to happen in parallel.

user16758: There is a very important rule for castling with one hand. In the Fide Handbook it is rule 4.1.

4.1 Each move must be made with one hand only.

The rule applies for all parts of the move, including capturing, castling and promoting.
Now for the reasoning, of why a move needs to be executed with exactly one hand: In case that you are making an invalid move there are specific rules on how to proceed after taking back the invalid move. As a general rule you have to move the first piece that you touched (rule 4.3.a). In case that you use both of your hands for castling, it would be pure luck to say that you need to do another king or rook move. Instead the order is clear. You first touch the king (and move it two squares). Then you touch the rook and move it to the other side of the king.
And why do you have to move the king first? Again it is to reduce any ambiguity. If you'd move the rook next to the king (without touching the king) you already completed a legal move. Castling now would be very weird as for the only time in chess a completed move could still be extended. Your opponent therefore could already make his move and you could still go: "I'm not finished yet, please take back your move". (In an extreme case you could wait for all the time on your clock before announcing that you are castling).
For these reasons it is important that you execute the castling in two steps (first king, then rook). The additional rule that you can only use one hand is not of practical importance as long as both parts of the move are clearly distinguishable. 

user4418: 
why is it illegal to castle with both hands?

Because the rules say so.
From the FIDE Laws of Chess -

7.5.4 If a player uses two hands to make a single move (for example in case of castling, capturing or promotion) and pressed the clock, it
  shall be considered and penalized as if an illegal move.


user3009: Chess rules are all about sequence.  If you use two hands, you don't know which happened first.  So you always use one hand.  First, you make your move, then you hit the clock.
As others have pointed out, the rook move alone could be one or two moves.  Maybe, you were just moving your rook, maybe you were castling.  The King move, however, does determine the one and only move.  There is no other King move that allows the King to move two squares.  By moving the King first, then moving the rook, there are no questions as to the real move.  The Rook, while being part of the move, is forced.  Because this is a sequence, you have to use one hand to play it.
As a side note, if you could use both hands, and you were playing with a clock, you could move the rook and hit the clock with one hand, and move the king with the other hand, and then there would be a sequence argument whether the king had moved before or after the clock hit, and thus whether it was a rook move or a castle.  By requiring one hand to do all three (king, then rook, then clock), there is no question as to what went on.  The king moved two squares first.  That defined the move.  Then the rook moved (with the same hand), then the clock was hit after the board was in a valid situation.  No arguments.

user9627: I know in general the rules say the hand you move your piece with has to be the one that hits the clock. If you castled with two hands which one would you use to hit the clock? This might be part of the reason for the rule saying you can only castle with 1 hand.

user12803: Disclaimer: this is probably not the answer, but it makes for interesting thinking. 
This reminds me of a string bet in poker. String bets are illegal, because they can be used to gain information – “I’ll see your $5”…  < watches opponent’s face> "…  and raise you $20”. 
By moving the king first, you are making what would otherwise be an illegal move (moving the king by more than one square) and cannot “string bet”. 
If you were to move the rook first, you might observe your opponent’s face, then quickly choose to move the king, claiming that you had intended to castle all along.

user17197: The official procedure to castle is (Schiller 2003:19–20 from Wikipedia):

first move the king with one hand and then move the rook with the same hand.

By using both hands the player can save time, as would by as using different hands for moving (like promoting a pawn to queen) or moving with a hand and hitting the clock with the other.

16705: <p>I was watching <a href="https://youtu.be/roBJYinLCPc?t=322" rel="noreferrer">the match between Ian Nepomniachtchi vs Hikaru Nakamura</a> and Nakamura used both hands to castle.</p>

<p>Why is it illegal to castle using both hands? What's wrong with that?</p>
 -1: <p>It's because you can't save yourself time by moving with more than one hand like that. Imagine one minutes where people did that. Castle would be over powered for time management reasons. Therefore, you castle with one hand, and the problem is avoided.</p>
 15221: <p>This is speculation, but I think it's because FIDE wants to reduce the difficulty of knowing whether the rules are followed. One must use the same hand to move the piece and hit the clock because FIDE wants people to not hit the clock before they finish their move, and if different hands were used for the two actions, it could be unclear which happened first. And if someone uses both of their hands for a move, then clearly at least one of the hands used in the move will be different from the one used to hit the clock, making it difficult to see whether the other hand completed its portion of the move before the clock was hit. And it's not just what is easy to see, but what is clearly distinct. If we're trying to decide which hand completed its motion first, that is a matter of perception. But if we're trying to decide whether two hands were used, that's a clear binary. It's easy for two players to have differing opinions as to which hand completed its motion first, but whether two hands were used is an objective fact, and if the players disagree, that strongly suggests that at least one of them is just straight up lying.</p>
 17210: <p>In chess, whenever you touch a piece, you MUST move that piece. If you were to begin castling using two hands, and then removed your hands, it would be unclear which piece you would be required to move. This is why it must be done with one hand.</p>
 6885: <p>It would permit a rather interesting ambiguity on the board.</p>

<p>Many have mentioned that the king has to move 2 spaces first, so that there's only one possible valid thing to do.  So let's assume we do this.</p>

<p>With one hand, move the king two spaces.  Once the king is in place (proving you intend to castle), the other hand grabs the rook and moves it.  Meanwhile, the first hand goes to hit the clock.</p>

<p>It is possible that the first hand hits the clock before the rook is moved.  Now you have a really funny situation where your hands are still moving pieces <em>while it is the opponent's turn</em>!  Needless to say, this could be a big deal, especially if the winning move is to take that rook.</p>

<p>To avoid this case, one needs rules which ensure that all pieces are in their final position before the clock is pressed.  Whatever rule FIDE chose, this is the key requirement placed on that rule.  Hands off the board when it's not your turn!</p>

<p>If we use <a href="https://en.wikipedia.org/wiki/%CE%A0-calculus" rel="noreferrer">pi caluclus</a> we can define a process where the clock press must occur after both moves are complete, regardless of which hand does what.  So in theory, one might define a different rule which permits two-handed operation.  However, in real life practical situations, such definitions are unreasonable. They work great with transistors and wires, and less well with human hands.  Instead, we simplify the process by not allowing things to happen in parallel.</p>
 16758: <p>There is a very important rule for castling with one hand. In the <a href="https://www.fide.com/fide/handbook.html?id=171&amp;view=article" rel="noreferrer">Fide Handbook</a> it is rule 4.1.</p>

<blockquote>
  <p>4.1 Each move must be made with one hand only.</p>
</blockquote>

<p>The rule applies for all parts of the move, including capturing, castling and promoting.</p>

<p>Now for the reasoning, of why a move needs to be executed with exactly one hand: In case that you are making an invalid move there are specific rules on how to proceed after taking back the invalid move. As a general rule you have to move the first piece that you touched (rule 4.3.a). In case that you use both of your hands for castling, it would be pure luck to say that you need to do another king or rook move. Instead the order is clear. You first touch the king (and move it two squares). Then you touch the rook and move it to the other side of the king.</p>

<p>And why do you have to move the king first? Again it is to reduce any ambiguity. If you'd move the rook next to the king (without touching the king) you already completed a legal move. Castling now would be very weird as for the only time in chess a completed move could still be extended. Your opponent therefore could already make his move and you could still go: "I'm not finished yet, please take back your move". (In an extreme case you could wait for all the time on your clock before announcing that you are castling).</p>

<p>For these reasons it is important that you execute the castling in two steps (first king, then rook). The additional rule that you can only use one hand is not of practical importance as long as both parts of the move are clearly distinguishable. </p>
 4418: <blockquote>
  <p>why is it illegal to castle with both hands?</p>
</blockquote>

<p>Because the rules say so.<br>
From the <a href="https://www.fide.com/fide/handbook.html?id=208&amp;view=article" rel="noreferrer">FIDE Laws of Chess</a> -</p>

<blockquote>
  <p>7.5.4 If a player uses two hands to make a single move (for example in case of castling, capturing or promotion) and pressed the clock, it
  shall be considered and penalized as if an illegal move.</p>
</blockquote>
 3009: <p>Chess rules are all about sequence.  If you use two hands, you don't know which happened first.  So you always use one hand.  First, you make your move, then you hit the clock.</p>

<p>As others have pointed out, the rook move alone could be one or two moves.  Maybe, you were just moving your rook, maybe you were castling.  The King move, however, does determine the one and only move.  There is no other King move that allows the King to move two squares.  By moving the King first, then moving the rook, there are no questions as to the real move.  The Rook, while being part of the move, is forced.  Because this is a sequence, you have to use one hand to play it.</p>

<p>As a side note, if you could use both hands, and you were playing with a clock, you could move the rook and hit the clock with one hand, and move the king with the other hand, and then there would be a sequence argument whether the king had moved before or after the clock hit, and thus whether it was a rook move or a castle.  By requiring one hand to do all three (king, then rook, then clock), there is no question as to what went on.  The king moved two squares first.  That defined the move.  Then the rook moved (with the same hand), then the clock was hit after the board was in a valid situation.  No arguments.</p>
 9627: <p>I know in general the rules say the hand you move your piece with has to be the one that hits the clock. If you castled with two hands which one would you use to hit the clock? This might be part of the reason for the rule saying you can only castle with 1 hand.</p>
 12803: <p>Disclaimer: this is probably not the answer, but it makes for interesting thinking. </p>

<p>This reminds me of a <a href="https://en.wiktionary.org/wiki/string_bet" rel="noreferrer">string bet</a> in poker. String bets are illegal, because they can be used to gain information – “I’ll see your $5”&hellip;  &lt; watches opponent’s face&gt; "&hellip;  and raise you $20”. </p>

<p>By moving the king first, you are making what would otherwise be an illegal move (moving the king by more than one square) and cannot “string bet”. </p>

<p>If you were to move the rook first, you might observe your opponent’s face, then quickly choose to move the king, claiming that you had intended to castle all along.</p>
 17197: <p>The official procedure to castle is (<a href="https://en.wikipedia.org/wiki/Rules_of_chess#CITEREFSchiller2003" rel="noreferrer">Schiller 2003:19–20</a> from Wikipedia):</p>

<blockquote>
  <p>first move the king with one hand and then move the rook with the same hand.</p>
</blockquote>

<p>By using both hands the player can save time, as would by as using different hands for moving (like promoting a pawn to queen) or moving with a hand and hitting the clock with the other.</p>
