In local co-op mode, players can drop in and out at will. When they come in, they appear next to player 1. This means that if you have a difficult area to get across, where moving all your co-op partners proves difficult, you can drop them out, then bring them back in once player 1 has crossed - they will appear next to him.
In Trine 2, if you are standing on top of two crates, it is possible to levitate the lower one, allowing you effectively to fly. You can't levitate an object you're directly standing on, but you can do it to one underneath. This allows you to subvert many of the game's puzzles.