I have a head-to-head campaign going on now, and it has been fairly stable. Two crashes in about 60 turns. One happened because my friend attacked an AI army in forced march stance that was stuck between two rivers. In that case, the system is unable to render the battlefield and crashes during the battle setup, but this happens also in SP campaigns. Another crash we had was due to my friend's system not being able to hand some effect during a battle.

We have a head-to-head campaign but we're not really playing for defeating the other player ASAP. We mostly play this so we can be the brains behind the AI in our battles while we build up our own empires. Has been great fun so far. As my friend says: the campaign plays out very differently from his SP experience. He is losing way more troops even in small skirmishes than he is used to when playing against the AI. I've had my share of battlefield surprises too.

By the way, a head-to-head campaign does not start in a default war with the other player. The campaign starts neutral. Once we even allied with each other. While we were allied we could not play the AI side in battles: just spectate. Once we canceled the alliance, we could play as the AI again.

This leads me to a question for the players who play a coop. Is it possible to get out of that default starting alliance?