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Instead of playing with a friend at your house at your house you could go to Campian then inviteone of your friends xbox live and communicate with them via headset pick the level and diffucultly then start i think it would be cool wat do u think.
Idea for alternate characters in Halo 3's multiplayerOnly click on this link if I tell you to.
judgeing by the co-op setup of Halo 2, it was going to be online but got scrubbed....so most likely in halo 3 they will have it
Levels on Legendary can take hours, I don't think it'd work with a disconnection every 15 minutes.