Coplaying is allowed, just make sure you keep by the rules for knowing the password to only one account per game world. For instance, if player A plays account A on world 1 and world 2, and player B coplays account A on world 1, player B cannot have another account on world 2, even if he isn't playing account A on world 2, since player B still knows the password to 2 accounts if he does so.