If you start a battle that will bring in reinforcements (either on your side or the enemies), when those reinforcements come onto the battlefield, the guest player is unable to see those ships. I have had one instance of the friendly ship showing up to the other player but it was off the battlefield (outside of the gird) and uninteractable for him. More than a bit frustrating when you're trying to do a battle together and since thats all the other player can do in coop its kind of a gamebreaker.