Remember that video from a long time ago that showed those two guys killing the Cyclops? This is the same exact spot, in the Desert of Lost Souls, behind that door that it broke through. It looks like there really is a co-op mode!
As for co-op there is a video that looks like a co-op but that also could be just 2 guys from same team in multiplayer
that was in a trailer where they first revealded multiplayer at first you see 2 people killing a clyclops teasing for co-op but then suprised with something else , also in the first 30 min video of the story there is a part where you cansee your multiplayer character http://www.youtube.com/watch?v=pYJ4A53UziQ&t=26m12s
hes there only for a momment but im sure he'll be seen more as you go deeper into the story mode.