One of my favorite things to do while playing Mortal Empires with a friend is to create supporting armies to cover each others weaknesses. For example, my friend and I played a Bretonnia/Empire campaign in which he sent an atillery army to follow my main force while I sent an aerial army to support his. This tactic came with two glaring problems:
1) Due to being turn 1 I was unable to initiate attacks utilizing the support. I would always need to get into position and then have my friend attack, or just group together and hope we got attacked. This wasn't as big of a problem as the second point, but remember he had a support army of only ranged units, so the fact that he was the one that needed to start the attack always put him in danger.
2) Because I could not initiate attacks, I couldn't take cities without my friend first razing it. This was the most annoying part of the game, even if it was a city I needed he would always be the one forced to make the attack.
I believe that a shared turn would easily fix the problem with working together and also make the Co-op campaign ACTUALLY Co-operative, instead of just single player with two people.