A co-player is somebody you trust who has the login name and password to your TW account.
You cannot both be on the account at the same time.....if one is logged in and the other logs in, it will kick off the one already in.
You usually communicate through skype for instant communication....as this is the key to co-played accounts. You can check to see if your co-player is already on the account.
You can have more than one co-player....so, for example, 3 people can play the same account.
Ideally, but not absolutely necessary, co-players are usually able to play at different times of the day.....so the account is active for longer periods.
Hawk