Thank you a lot, Firo!

Right now I think that the brother / sister idea is better. We could have them work together in a level (one at a time) in order to solve puzzles, defeat monsters, etc. The big sister would be able to reverse the gravity from the very beginning and the little brother (and his sister as well?) would learn different abilities as he advances in the game.

This would allow us to mix puzzles and arcade. Here's a simple example: the exit door is locked and guarded by a monster. The sister pushes a button on the ceiling to open the door, the monster gets out, the brother pushes a box that locks it into a certain area of the level, allowing both brothers to reach the exit door, etc. What do you think?