Get Shank from Xbox Live Download, there's a specific 2 player co-op that must need 2 player one.

now come to think of it, there isn't much co-op game, especially ones that support split screen.

Mass Effect 2's combat is a bit similar to Gears of War, but it is single-player only.

Fable 2 you can drop in a 2nd player in single-player game, it's a hack n slash RPG but you can use guns.