Portal 2 is Split-screen Co-Op.
Here: Just "Find: Split" And you'll find all the Co-op Xbox 360 games that have split screen co-ops.
http://en.wikipedia.org/wiki/List_of...r_the_Xbox_360
GOW does have split-screen co-op.
BTW, ODST is very good for a Co-Op, but really only has about 12 hours of gameplay time to 'beat' the game. And Left 4 Dead 2 is good as well, but I find it more enjoyable on PC than on Xbox.