This seems like it needs a lot of work but the idea could work well. Several people already mentioned the air control. I would also make the jump a little less floaty (increase gravity values) and move the camera back a little bit (the characters jump very high, so it becomes hard to see things with the camera this close). If you can build levels in such a way that the characters really need to help each other to reach the end, this could be a really good game.