This has been posted by several people in this forum. My wife and I like playing this game, but it is soooo annoying that only player 1 gets the achievements when you pass the level.
I really wish the developer would send out an update to fix this. I play a lot of co-op multiplayer games and have never had this problem before with those games (Scene it, COD5, Lego star wars, etc.)
But, like most posts, this will probably be ignored and never fixed.... arg....