Although having a second gun is always handy, Army of Two
's primary gameplay mechanic focuses on something referred to as "Aggro" and isn't much different than a common tactic used by most MMO players. The idea is to have one guy divert enemy attention to himself while the other sneaks up behind them to attack or complete some other objective. The bigger and longer the distraction, the greater the benefit.
In addition to basic cover-fire techniques, the two can also use more advanced tactics like using each other as a step-up ladder or a moving-cover tactic where one guy holds a car door as a shield while the other shoots. There's even a part where they'll have to perform a tandem skydive; one guy steers the chute while the other snipes enemies below.
For the most part, Army of Two plays like a tactical shooter. The game can be played as either a single-player game or with a friend. When playing solo, you take control of one mercenary while the A.I. looks after the other. Overall, your allied A.I. is decent and follows orders pretty well. Of course, there are occasional, "What the hell??" moments, but otherwise there are few glaring problems.
The story spans six campaigns. In each mission, you are given a primary goal as well as secondary objectives. Completing missions earns you money which is used to upgrade or purchase weapons. Each mission also has a secret side-mission that will earn you a little extra money. Missions are fun, though incredibly short. The game can be completed in a matter of hours and is only worth going through again if you can find a friend to play co-op with.
Army of Two gives you plenty of multiplayer options. You can play split-screen with a friend in the same room or jump online. If working with others isn't your thing, there are also three competitive multiplayer modes. Although most of the modes are basically just take-offs of pre-existing multiplayer game types, Army of Two adds a different take. Games are played with two-man teams, which adds a different dimension to otherwise run-of-the-mill modes.