You should allow it, but tell him and the other player that they have to replicate the dance just right. Out of character.
On a more serious note, I think that when you get into that kind of custom spell, you've gotta just throw any idea of balance you had out of the window. You'll be losing an entire PC worth of power for however long it lasts in order to perform this fusion, so I think they should be compensated. One idea is to simply make the two characters into a gestalt of their two classes, with two more levels of progression to offset the loss of an independent turn worth of actions.
Just my two cents.
Either start a different game with different rules that let you do this, or introduce the group to Teamwork Feats?