I just noticed that whenever I push a series of crates with my shadow on top of it, my shadow will slide in the direction I'm pushing [EDIT: Only works when moving to the left ]. The crates themselves don't slide .
This also happens the other way round . Here's the situation:
I would like to suggest that, to add more puzzle elements, the player or shadow can only slide single crates. Two or more crates next to each other should not be pushed by a single character.
I'm not sure if having both characters pushing two crates next to each other should result in movement of the crates. This will, however, add extra puzzle elements and an extra filter to ensure both characters are needed to progress (in a level).
Good luck debugging .