Yeah, it's dumb how CoD4 allows for two-player split-screen offline, but not online.
Anyway, here's your best bets for split-screen play off the top of my head:
-Halo 3 (allows for online and offline co-op AND deathmatch)
-Gears Of War (allows for online and offline co-op AND deathmatch)
-Samurai Warriors 2 (offline co-op)(horizontal split-screen)
-Dynasty Warriors Gundam (offline co-op)(vertical split-screen WHY KOEI WHY)
-Warriors Orochi (offline co-op)(horizontal split-screen; limited to same characters for both players)
-Earth Defense Force 2017 (offline co-op)
Out of the three Koei games, Samurai Warriors 2 is the best for co-op, IMO. The problem with Orochi is that both players have to use the same three characters on their teams, and Gundam uses an annoying vertical split-screen.
You can also always go the old-school route and play some XBLA games like Contra, Double Dragon, and TMNT. Don't forget about the co-op mode in Splinter Cell 3 either.