BlueMoon writes "It has now been confirmed that Halo 2 will not include support for co-op mode on Xbox LIVE or LAN play. The co-op mode will remain for offline play only. Bungie made all attempts to implement it however with the complexity of the game it was simply impossible to have a worthwhile co-op experience."
You may not be able to play together, but as consolation, check out the Halo 2 Ads
pointed out by an anonymous reader.