When two dancers perform together, there is an understanding that each has practiced the routine, they will keep count together, act together, angle themselves to move and dance as one. Any major deviation from the planned routine can lead to the whole presentation falling apart, and so they both must be as perfect as possible. They will practice to ensure they meet the requirements to perform as instructed.
At the same time, minor deviations can be covered up. The two can respond to the other and ensure the scene progresses without pause. They anchor each other into the role, they perfect each other. Something that we took to heart.