user15543: One consistent feature of chess over the centuries is that each side has had one king only. The rules of promotion had to be tightened up to prevent promotion to a second king.
However, if promotion to kings was allowed, a ruleset would be needed to deal with checkmating multiple kings. In my view, such rulesets would need to be, relatively speaking, practical for tournament and OTB chess. By practical I mean it would be relatively easy to learn and to play.
While there known fairy chess conditions, like Rex Multiplex in which all kings are mated at the same time, they are impractical for real-world play. These new rules would need to deal with checking two kings at the same time, forks, pins, stalemate, and promotion circumstances, among many possible things.
What would a practical ruleset for dealing with multiple kings if king promotion was allowed? All regular chess rules apply, including check and castling.

user8201: The simplest and the smoothest rule for play is to say that if a player has multiple kings, they are not royal, i.e. they aren't subject to check and can just be captured. When a player is reduced to having only one king to lead his army, the normal rules apply.
The mate-one-to-win rule means it is disadvantageous to promote to a king, while mate-all-to-win can be absurdly difficult to achieve. The rule above seems a practical middle ground.

15543: <p>One consistent feature of chess over the centuries is that each side has had one king only. The rules of promotion had to be tightened up to prevent promotion to a second king.</p>
<p>However, if promotion to kings was allowed, a ruleset would be needed to deal with checkmating multiple kings. In my view, such rulesets would need to be, relatively speaking, practical for tournament and OTB chess. By practical I mean it would be relatively easy to learn and to play.</p>
<p>While there known fairy chess conditions, like <a href="http://www.matplus.net/milanvel/FairyKings.pdf" rel="nofollow noreferrer">Rex Multiplex</a> in which all kings are mated at the same time, they are impractical for real-world play. These new rules would need to deal with checking two kings at the same time, forks, pins, stalemate, and promotion circumstances, among many possible things.</p>
<p><strong>What would a practical ruleset for dealing with multiple kings if king promotion was allowed?</strong> All regular chess rules apply, including check and castling.</p>
 8201: <p>The simplest and the smoothest rule for play is to say that if a player has multiple kings, they are not royal, i.e. they aren't subject to check and can just be captured. When a player is reduced to having only one king to lead his army, the normal rules apply.</p>

<p>The mate-one-to-win rule means it is disadvantageous to promote to a king, while mate-all-to-win can be absurdly difficult to achieve. The rule above seems a practical middle ground.</p>
