First, EA has patched Red Alert 3 to version 1.04. The best way to update is to use the auto-updater in game, but if you want to download the patch manually, you can do so here (note that it is about 825MB larger when downloaded manually).
Also, Amir Ajami, a producer on Red Alert 3, granted an interview to 1up about the coop campaign (which has hitherto been kept under wraps). Here's a piece:
1UP: So when you're in the co-op scenario, does the second player control a duplicate force of guys and tanks?
Amer Ajami: It's rarely, if ever, duplicate. They're both tailored to complement each other; for example, the mission we're showing in Leipzig is a Soviet mission that's halfway through the campaign. It's an island-hopping mission where one player has mostly air forces and the other player has mostly ground forces. So you'll have to use the ground forces to knock out the anti-air units on an island to let the air forces come in and clean up the rest of the area or take out any enemies that are strong against your own ground forces. So there are a lot of complementary force situations that you'll play through. In later missions -- the more base-versus-base ones or the last, or second-to-last, mission of any given campaign -- where, at that point, you've unlocked most or all of the technology tree, then you'd be in a situation where you and your buddy both have a Mobile Construction Vehicle and you both can essentially unlock and build whatever you want, and throw whatever you want against the enemy.
You can read the full thing here.