Well, the nice thing was you weren't fighting alongside your partner most of the time. You'd both have seperate mission objectives that would intersect at select points, requiring the action of the other player to advance, covering them from enemy fire, hitting a switch, that sort of thing. It wasn't a special multiplayer mode either - that's how the game is going to work in single player mode...just now with the standard offline AI. The whole deal was implemented very well, you could play offline for a few levels, hop online, and keep progressing through the game with your buddy.