A look at how Mercenaries 2 does co-op
IGN has a pretty decent writeup detailing their hands on time with two player co-op in the explosive Mercenaries 2: World in Flames. Online co-op is presented pretty standardly, allowing two players to connect seamlessly where one player enters the others' world a la Crackdown. Meaning that any progress made during co-op only counts towards the hosts' game, but any weapons or money earned by the other player can be taken back to their game. What's nice to see too is the fact that the game doesn't "scale" the action based on co-op, which means that having a co-op buddy by your side makes dishing out the destruction a whole lot easier. Make the jump to get Mercs 2 learnified.
