Army of Two's focus has always been cooperative gameplay, EA Montreal's Eric Chartrand is responsible for designing the competitive multiplayer aspects of The 40th Day. In an interview with with That VideoGame Blog, Chartrand notes that one of the big changes for the AoT sequel is to adapt the multiplayer aspect, and make it play differently from the campaign mode. Whereas the story can feature designed bottlenecks (such as doors that require both players to open), multiplayer cannot, simply because humans play differently than AI. "The toolset we use in single-player has to be different from the one we use in multiplayer," Chartrand explains.
Interestingly, Chartrand notes two games as his main inspiration for The 40th Day's multiplayer offerings: the decade old Counter-Strike and the recently-released Killzone 2. "What works in Counter-Strike is that you don't get progression," a stark contrast to the level-up gimmick common in most online shooters. "So why it works is because the gameplay in it. The second-to-second, the movement of the character, the shooting, the intensity of the combat is still fresh, even ten years after. So that's why we wanted to craft an experience that is as close to this as possible."
Chartrand has even more praise for Guerrilla's FPS. "Killzone 2 has great maps, probably the best maps out there," he admits. However, what inspired Chartrand the most appear to be the game's community features, particularly its stats-laden website. "This link between stats, profile, friends, leaderboard ... all the community aspects, I think the Killzone guys did a very good job and we strive to imitate them."