You're thinking of how we used to play DOOM together, dumbass. Co-op in Halo2 can only happen split-screen.

And Titus, gruesome is referring to Outskirts, the level that starts with your pelican crashing. Immediately after you go through the first doorway that leads out to the courtyard, there is a light above your head. You can crouch-jump onto it and then jump up even higher onto a flat roof. Jump across the gap you will find and crouch-jump to the top of the wall you will find on your right and you're basically on the top. Run to where the roof becomes pointed and jump across the alleyway to the other roof and you can basically have access to any rooftop in the game. Some parts (getting across the broken bridge) require grenade jumping, but it is possible. I saw a video showing how to do it, I'll try to find it for you.