Then nothing... if you have two accounts registered on an xbox, when you sign into xbl, it should say to sign in as a guest to the dominant account, or as a different one.
No, you do not need different accounts, once you are signed into xbox live, just go to the pre game lobby and have someone log in as a guest, and you do need a different account if you plan to play ranked games, but you can still play the training games with guests.