What about cooperative play?

A: Xbox Halo supported a split-screen-only, two-player-only, one-Xbox-only cooperative play mode that, while quite a lot of fun, was a relatively late addition to the game. It was decided during the development of the PC port not to create a splitscreen cooperative mode for PC Halo, as this would not match the expectations of most PC gamers. Gearbox intends to attempt adding a networkable cooperative mode to the game, but not prior to the initial release. Westlake has said that if Gearbox is able to support such a mode, they would also attempt to support it, as long as it did not break multiplayer interoperability.

The difficulty in supporting such a mode is that the single-player levels of Halo are far larger and more complex than the multiplayer levels. Cooperative was achieved on the Xbox by creating two, half-height view ports into the same game world. The amount of data that would need to be moved over the network connection in such a cooperative model on the PC would be prohibitive.