Think of it like the old game with the ice and the fire. You have to work together to get across. Maybe have limited times you can change environment. You have only two characters like it is now. It's very easy to understand and introduces the player/s perfectly. It's not about the mechanics being difficult to master but balancing the game to increase difficulty and giving every player the time to get acquainted. You can watch a video from Game Maker Toolkit named who should be awesome if you want to see more.