Focusing on cooperative strategies, Army of Two's main feature is the necessity to use coordinated teamwork to accomplish the game's goals. While the game is meant to be played with another human as a partner, a "Partner Artificial Intelligence" (PAI) is also included and programmed to follow the player's strategies. Dependence on a partner (whether human or PAI) is so pronounced that most objectives are impossible to complete without it.
The PlayStation 3 and Xbox 360 versions of Army of Two were one of the first games for the console to be region-locked for online play. EA claimed that the region-locking was to prevent network lag caused by players from multiple regions, and to prevent Asian region console owners from playing the US and European versions of the game, as the Asian version has been censored to meet certain requirements (notably, the shooting of already dead bodies in the game).
The player can upgrade their armor to make it better and stronger but these cannot be purchased and are unlocked as the player progresses through campaign mode. The players start with Basic body armor, upgrade to Medium after completing the Afghanistan mission, and are granted Heavy body armor after the Aircraft Carrier mission. Players can also unlock and purchase new face masks that the main characters wear.
The game allows weapon customization, bought with money earned as the game progresses. Such elements as new barrels, stocks, forend-mounted vertical grips, and extra-large magazines and ammo drums that can be swapped out to give the weapon a unique look and superior statistics. Other modifications include suppressors, a gun-shield, an under-slung grenade launcher, or an under-slung 12-gauge shotgun.
A firing range feature was proposed that would allow the player to test out their newly customized weapon, however it was removed as it did not meet the developers' standards.
The original game boasted three different drivable vehicles: a Jeep, a main battle tank, and a hovercraft. Gameplay was designed so that one player would drive and the other would fire a secondary weapon like a ring-mounted XM312 heavy machine gun (Jeep and hovercraft) or a 120mm cannon (the tank). The Jeep and tank were removed from the final product due to "pacing issues" and the hovercraft was used only in the China and Miami levels.
Multi-player (or "Versus" mode) allows up to four players, with two on each team (similar to the co-op structure of the main campaign). Both teams will compete over objectives on the map such as assassinating a VIP or destroying an objective in order to earn the most money for their side. At the same time, they will also be forced to deal with the opposing team in order to secure their objectives.
Multi-player for the game is currently region-locked. Players with the European, American or Asian version of the game cannot play with each other.
There are 3 multi-player game types: Warzone, Bounties and Extraction.
- In Warzone, the primary and secondary objectives are randomly generated and they may vary greatly. One team might need to get to a certain place and defend it, while the other team tries to destroy it, shoot down the chopper, blow up a jeep with its gunner, transport wounded soldiers to the extraction point, assassinate V.I.P's, etc.
- In Extraction the objective is to rescue randomly-spawned V.I.P.s or P.O.W.s to a safe location.
Also every map contains cases of intelligence documents that the player can seek out for extra cash.
Character customization is not possible in the multi-player portion of the game but the player can buy preset sets of weapons, upgrade the characters body armour and the amount of ammo he can carry. Players need to be careful while shopping though, since the team with the most cash in the end wins/
On the May 27, 2008 edition of X-Play from G4 TV, it was announced that two new maps would be available for download on May 29, 2008. It contains a new Co-op map (fighting and eliminating a dangerous Russian militia group based in Kiev's subway system), an expanded Versus map (China Canal Lock), a more dynamic and interactive environment, more destructible objects, and 6 new "secret" achievements. There's also an alternative ending to the game in which the players can take on Dalton in a more dramatic boss battle at SSC's Miami headquarters in which Salem and Rios have to lock him in a vault.
Implemented multi-player manoeuvres include:
- Step Jump/Pull Up: One person stands up against a wall and holds his hands out while the other person gets lifted up. The second person can then reach down and pull first person up.
- Back to Back: Both players go back to back to cover each other's non-armored backsides. Unfortunately, it can only happen at certain scripted moments.
- Driving: In the Miami level the team can use a hovercraft that needs one player to drive and the other to man the machine-gun turret. The original game also boasted a Jeep with a gun mount and a Main Battle Tank, but they were eliminated by the developers because it slowed down the gameplay.
- Kudos: Both players can do a range of emotes from slapping each other to playing air guitar. Also characters will communicate a lot and talk trash to one another. For example, one of the random emotes that Rios will occasionally say after being healed is "Where did you learn first aid, butcher school?" Players can also show anger to their partner by smacking them or head-butting them.
- Healing: When one player is injured, the other player must run over to him and either drag him to a safe area or heal him in that spot. Both players may shoot while one is being dragged, but during the healing only the injured player has the ability to fire at the enemies. The injured player also has a "bleed-out" bar, which limits how much time they have left before they die from their injuries. This bar decreases depending on the difficulty played at or if enemies continue to shoot them. Pre-release versions had the critically injured player's spirit running from a bright light while the still living character mashes buttons to perform CPR in an attempt to revive the wounded player.
- Aggro Meter: This is a pendulum-like gauge that indicates which player is the focus of the enemy's attention. Whichever player has the highest Aggro rating will attract all the enemies towards them. This allows the other player to become virtually invisible, and thus sneak up to enemy positions. If the aggrometer is full to one player they can initiate an "Overkill" mode, which the player with the most aggro goes into a fury doing x2, x3, x4 etc. shooting damage to the enemies, in this mode bullet time is initiated and the player cannot crouch, but doesn't have to reload either. The other player without aggro becomes transparent and moves faster, they are recommended to use hand to hand combat while in this mode. Both players have 15 seconds before all aggro is drained and both players are equal.
- Feign Death: If player one is about to die, he can fake his death. All the aggro will be transferred to the second player. Then player one can get up and attack enemies from behind (if walked past). Though this can be done multiple times, the same enemies will fall for it only once.
- Co-op Snipe: Both players get into a sniping manoeuvre to double their power or take out two targets at the same time. To ensure they fire simultaneously, an automated countdown will be initiated.
- Weapon Swap: Players switch weapons with each other to diversify their arsenal.
- Riot Shield: One of the players picks up a riot shield or a car door and both players hide behind it. While one player holds the shield, the other player has the ability to open fire at incoming enemies. The player that carries the shield also has the ability to hit enemies with the shield using melee attack.
- Parachuting: In some missions Salem and Rios will be traversing the skies via parachute. Then one player controls the parachute, while the other shoots targets on the ground.
Upon its release, Army of Two received a mixed, though mostly positive, critical reception.
Army of Two: Dirty Money, written by John Ney Rieber and illustrated by Brandon McKinney, is a 2008 graphic novel which follows Rios and Salem through some of their earliest missions together working as private military contractors up to the War in Afghanistan in September 2001. The plot follows the corruption of the private military company that they work for as private military contractors. Rios and Salem work together as an 'Army of Two', trying to stay alive and uncover the conspiracy within the company that employs them.