How can I play on xbox live with two people on two different controlers using only one gamer profile?

For example i will see people using the same gamer profile like (grsofwr15 and (1)grsofwr15). I don't understand how they do this and i want to find out how really bad. Im playing gears of war and i don't know if it is just the game or what.|||You can press Y I think when you choose Versus in the main menu. It will let you choose another gaming account so two people can play online. Just choose Guest and you are ready to play.|||Have them press the X-box button and have them sign in as guest under ur profile