Pappenheimer: thank you for the link. Don't worry, this is a more general thread so anyone can post here; we are thankful for all the ideas.

I agree that we shouldn't give too many abilities to the brother / sister; the new powers should be the icing on the cake. Using the two characters in the same level allows us to add many arcade elements to the game: we can crush monsters using boxes, open or close gates that lock or release the monsters, etc.