GameSpy: How To Sneak Friends Into Your Single-Player Games - Page 1
Doom 3 on the original xbox has system link co-op! Using 2 consoles you can explore a slightly altered campaign with a friend, because the mirror scene scares you. : r/originalxbox
DOOM 3 - Multiplayer Gameplay (FFA) - YouTube
Doom 3 (Xbox) - Co-op Intro - YouTube
Will Doom Eternal have co-op or splitscreen? | Shacknews