Microsoft confirmed today that Halo 2 will not contain online or system link support for the game's co-op single player campaign. Fans wanting to play through the game's solo mode together will have to do so in the same manner as the original Halo by utilizing two-way split-screen. Some good news regarding the offline co-op mode however is that Halo 2 will support widescreen which allows the game's 16:9 aspect ratio to translate into a virtual full screen for each gamer as they play in the vertical split-screen.
While the reasoning behind the omission of the online/LAN co-op mode has not been officially revealed from either Microsoft or Halo 2 developer Bungie, some online sources are stating that it is due to the complexity of the game and the network code.
News source: GameDAILY