Why is a tandem harder to ride uphill than a single rider bike?
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.