Everyone climbs at their own best rate and cadence.
On a tandem, both riders have to ride at the same rate and cadence, which will be closest to those of the strongest rider.
So neither of them are climbing at their best rate and cadence.
On a tandem with very well matched riders , they are in fact quicker, largely due to the draughting effect of the two riders being so close together.
On the other hand, if the hill is so steep that the riders would have to stand, it becomes a matter of rider skills.
Many tandem riders struggle to master the two-person coordination of efforts required to make the transition to the standing position.
bicycles.stackexchange.com/users/8273/andy256
When climbing a hill on single it is normal to rock the bike side-to-side as you climb.
This rarely matches exactly for both riders.
It's similar to the way people run slower in a three legged race.
bicycles.stackexchange.com/users/13745/Jeff Bell
Coordination:
When riding in a group of singles, it can be hard to stay together on a hill climb, because everyone climbs at different paces.
It depends on so many factors, weight, gear ratios, preferred cadence, getting out of the saddle when it feels comfortable and so on.
Being forced to climb faster or even slower than you'd like to can upset a rider's rhythm.
On a tandem it's more likely that your hill-climbing rhythm will be awkward.
If the captain wants to stand up, the cadence becomes more "stompy", leaving the stoker feeling they're not able to put much power in.
If the stoker stands, they can end up rubbing their face on the captain's back.
If they both stand, then the tandem can be hard to control unless the captain is much heavier and stronger than the stoker.
Power-to-weight ratio:
A tandem with two light riders feels similar in pace to a single with a heavy rider.
Most heavier riders are slower on climbs.
Lack of aerodynamic advantage: The Coordination factor above is more than compensated for on the flat by a tandem's aerodynamic advantage: 195% power for perhaps 120% aerodynamic drag.
However, air resistance is negligible on a climb.
It may be that other sources of drag are not reduced so much.
For example: Rolling resistance may be similar to two singles due to the similar weight.
bicycles.stackexchange.com/users/5337/James Bradbury
In addition to the stated answers there is also a psychological reason: the Ringelmann effect .
The more people work together on a task, the less each individuals' effort unconsciously (even if everyone thinks he does his best) becomes.
This was first observed on rope pulling and the decrease in performance
it is not explainable by synchronisation inefficiency alone.
This effect applies even for groups of only two people: (Ingham, Levinger, Graves, & Peckman, 1974) attempted to repeat this experiment in 1974 because they were unsure of the early methodology and descriptions of Ringelmann's earlier study.
Had groups of 2, 3, 4, 5 and 6 perform the rope pulling task.
Groups of 2 performed at 91% of potential, 3 at 82%, 4 at 78%, 5 at 78%, 6 at 78%.
Source
bicycles.stackexchange.com/users/10944/Konrad Höffner
It is not harder to ride uphill if your captain and stoker are in perfect unity .
I ride a tandem with my wife a lot, and I find it way easier to climb hills with a stoker than riding single.
If you have the proper communication, then it's literally two people riding one bike, which is double the power of one person riding one bike.
The real disadvantage of riding tandem, is drag , two people and a bigger bike don't cut though the air like one person on a compact frame does.
Which is why tandem track-racing times are typically a couple seconds slower than single rider times.
Well, that and the fact that tandem track races are typically for disabled people, like at the paralympics, where the stoker is disabled, and the captain isn't allowed to be a pro racer.
But on hill climbs, where aerodynamics isn't a factor, tandem racers annihilate single racers on the hill climbs.
Some pairs of people may struggle on a tandem because their riding styles are different, as others have mentioned already this only matters if you're doing something like riding out of the saddle, on a tandem bike
you can't be two minds, you have to ride as one mind.
bicycles.stackexchange.com/users/14392/ShemSeger
With equal power to weight ratio, why is a tandem bike harder to ride uphill? I have friends whose Cannondale tandem bike weighs the same as their single rider bikes combined. But they find it easier to climb hills (compared to the bunch) on single rider bikes than on the tandem. They are experienced tandem riders.
Why is a tandem harder to ride uphill than a single rider bike?
bicycles.stackexchange.com
<tandems>
bicycles.stackexchange.com/questions/25881
bicycles.stackexchange.com/users/13433/Phillip Ngan
It depends on so many factors, weight, gear ratios, preferred cadence, getting out of the saddle when it feels comfortable and so on. Some pairs of people may struggle on a tandem because their riding styles are different, this only matters if you're doing something like riding out of the saddle, on a tandem bike. On a tandem with  very well matched riders , they are in fact quicker, largely due to the draughting effect of the two riders being so close together. In other words, the speed of the tandem depends on how well matched the two riders are. Tandem bikes require the riders to work together. Where riders are well matched and cooperate, they can be much more effective on hills than single riders. When the cooperation stops, a rider standing up, changing cadence, wobbling or acting erratically can upset the balance and counteract the effort of the other rider.
Tandem bikes require the riders to work together. Where riders are well matched and cooperate, they can be much more effective on hills than single riders. When the cooperation stops, a rider standing up, changing cadence, wobbling or acting erratically can upset the balance and counteract the effort of the other rider.