Gamespot on AW2 wrote:

In a tag battle, you get to determine which CO is taking the lead and plan your strategy to take full advantage of both CO's powers. For example, the inexperienced but wealthy CO named Colin has weaker units but can purchase them for lower-than-usual prices. You can put him in the lead whenever you need to buy reinforcements, and then switch to a tougher CO, like Max, in order to dish out the damage.

it adds a slew of new twists, including tag-team matches, dual-screen battles, the real-time combat mode

new units like the stealth fighter and the megatank

the megatank, a slow but horrifyingly powerful vehicle that can cut through medium tanks easily, and significantly outguns even the neotank from Advance Wars 2.

By default, the computer will control what happens up top, and actually does a very competent job of it. If you'd rather not relinquish control to the computer, you can personally take command of the action on the top screen