Basically you're allied, can't declare war on each other. So you're up against the AI, which to be honest I really liked.

A bit like having an ally who throughout the campaign isn't an idiot and will stay by you through thick and thin, and of course since you're communicating with them it's much easier to plan your strategy and tactics. It's really a blast, but of course it does take some time and coordination to make happen, so they can be very hard to complete. And there were de-sync issues.