Army of Two follows the story of Tyson Rios and Elliot Salem, two ex Army Rangers turned contractors for SSC, a Private Military Corporation. They embark on missions ranging from the caves of Afghanistan all the way to abandoned villages of China. As they progress, they realize that the life of a PMC at SSC might not be as clean as they thought it was.
The game focuses on cooperative two-man missions and customizable weapons. Players have access to a variety of firearms, including pistols, shotguns, submachine guns, rocket launchers, light machine guns, sniper rifles, grenade launchers, and assault rifles. The arsenal can be modified with extra magazines and new barrels, suppressors, grip handles, and different plates that change the look. Players fight alongside either a live or AI teammate using two-man strategies as part of the deadliest military outfit a government can buy.
Players can perform various actions together, such as lifting each other up ledges, healing, covering each other's backs, combining driving a vehicle and shooting, feigning death, swapping weapons, protecting each other with shields, performing dual parachute jumps, and going "aggro" to attack the enemy's attention.
The multiplayer mode is available for four players, operating in two teams based on the co-op structure of the single-player game. Players must complete objectives divided over three modes - Warzone, Bounties, and Extraction - while dealing with the opposing team at the same time.