Take too much aggro, however, and you could end up under fire so heavy that your chances of survival grow slim. Aggro is also affected by your position and the size of your gun, making this one game where switching to a small, quieter side-arm to do the actual killing often makes a lot of sense. Sniping, for instance, can be useful, but the sight of a sniper rifle is practically an aggro magnet to those nasty terrorist types on the field.
The key to Army of Two, then, isn't just shooting straight but managing your aggro so that both you and your comrade can function effectively. Not only are your enemies too numerous and too tough to tackle with a more head-on approach, but certain troops come heavily armoured or will lurk in shielded machine gun emplacements, making it imperative that one or the other of you gets behind them.
On top of this, a number of secondary game mechanics need teamwork. There are sniper targets out there that need simultaneous shooting, plus areas where one partner gives the other a leg-up, then in return gets pulled up into position. At times, one of you can pick up a shield to give mobile cover to a partner who does all the shooting. There are also back-to-back sequences where you guard each other's rears while enemies attack in slow-motion - it's a shame these only occur at preset moments.
Even the health system asks you to work together. The game uses a Call of Duty style fade-out when you're low on life, and if you fail to crawl to cover and rest you'll find yourself collapsing on the floor. At this point your partner needs to race into the fray and drag you to safety before administering a little much-needed medical assistance.
Obviously this all makes sense in an online co-op context, but what about those of us who want to play alone? Luckily, EA has clearly invested a lot of time and effort into its PAI or Partner AI system. With you playing one merc, the CPU takes the other and tries to support you in whatever you're doing, using the environment and any tools at its disposal as effectively as it can.
It actually does a fairly good job, and you can help it do a better one not by issuing orders, as in squad-based games, but by instructing it to adopt hostile (aggro raising) or passive (aggro losing) holding or advancing behaviours. Sure, there are times when the AI goes skew-whiff - dragging you to the worst possible cover position under heavy enemy fire when you're hurt, or failing to make use of all the aggro you're busy building - but a lot of the time he does his job sensibly and with just the right amount of management.