Only 2? You make that sound like a bad thing.
I know Halo supports bringing 4 people online on a single box, and only 2 for the coop campaign. Any other games support more than bringing 2 people on? CoD3 supported 4 I think, but not any of the others. Gears supports 2; Army of Two suppots 2. Most other games don't support any split screen (sadly). R6V1 and 2 support splitscreen, but not when going online. Marvel Ult Alliance does not support any hotseat, despite having the exact same restrictions as a hotseat game when online.
I'd actually like to find more games that support more than (or at least) 2, but the only one I can think of is Halo 3.
More on topic, L4D is a GREAT coop game because the max team size is only 4, and in versus total size is 8. You can have fun if it is just you and a buddy playing through a campaign, with 2 bots to help out. It just gets better when both of you then hop online as well.