"If you've stood up to a bully and told him he was gonna get a beating, a clever response from him would be "Oh yeah? You and what army?" If you'd responded, "Um ... me and this guy," you probably would have been laughed into oblivion.
In Army of Two, that's all you get ... you and a buddy as private military contractors. The game is completely built around co-op, which the developers frequently found themselves enjoying in games like Halo and Contra (shout out, old school style!) so why not make a game that's all about co-op gameplay? Your buddy can be sitting next to you, via splitscreen, or they can be sitting in Sydney, Australia, via the online co-op. Two's company, three's a crowd."