In particularly hairy situations, Rios and Salem will enter back-to-back mode. In this mode, time slows down, you're granted infinite ammo for a few seconds and you can spin around splattering enemy soldiers to your heart's content. We were a bit disoriented at first and had trouble with our partner spinning too fast and blocking our view - we were spinning at the perfect speed - but we think we could get the hang of it with minimal practice.
Another example of teaming-up came at an early junction in the game, Rios ripped off a car door (somehow) and we got behind his makeshift shield. The two of us became a portable mini-fortress - at least until the car door got blasted into a pile scrap metal by about a dozen terrorists. Utilizing clever strategies like this are essential to staying alive in the tough-as-iron campaign.
At opportune times, you can enter dual-sniper mode. At first this mode will confuse the heck out of you by putting all the screens in different places. Once we figured it out, it seemed to be an effective way to shoot two dudes/things at the same time.
The most awkward element of co-op is the fallen teammate revival system. In many other shooters, a pat on the back or your teammate saying something like, "shake it off" is enough to remedy being pumped-full of bullet holes. But Army of Two is based in reality, and apparently in reality the best real way to fix a bullet wound is to stuff a tampon into it. The tampon insertion is a button matching mini-game that both players play. Presumably, the live player is trying to figure out which end of the tampon is supposed to go into the hole, while the terminally injured player is hitting buttons to mentally prepare himself for the humiliation his partner is about to put him through. There is also a CPR mini-game that plays the same way - it's not quite as interesting.
Even the multiplayer matches are team affairs. Four players split into teams of two and compete to accomplish mission objectives the fastest. You'll be able to attack the other pair and force them to respawn, but there will also be sets of AI controlled soldiers running around the battlefield attempting to stop both teams from reaching their objectives. The overall goal: to earn the most cash (often by blowing-up objectives), of course, just like real life mercenaries.
Rios and Salem will be teamed-up for every mission, so that's just a few of the ways they'll be relying on each other. They can also drag each other to safety, distract enemies that can only be injured from behind and dole out hi-fives or bitch-slaps depending on the occasion. We'll see what else they have in store for us when the game comes out in early 2008.