If a cube was scrambled and two speedcubers could each take turns performing one quarter twist without any sort of communication between each other, would it be possible to solve the cube in a reasonable number of twists and in a reasonable amount of time?
The two should be using the same method (or at least recognise which move sequence is being used for a particular situation by seeing the first move).
It can get fun if one person doesn't know the right move and makes a guess. The second player then keeps undoing it until he makes the right guess.
There is however a way to sneakily communicate. You could agree that if player 1 does a wrong quarter turn, then player 2 does a half turn of that face to show it is wrong, player 1 does another quarter turn of the same face, at which point player 2 can do the move that should have been done in the first place.