How do have two players with their own gamer tag on-line with one X-BOX?

I have one x-box and one on-line account but want to put two gammer tags and be able to play together on-line. Anyone know how I can do this with out purchasing another account.|||On certain games, you are able to sign a guest in under your gamertag, if you want two different tags, then you must purchase another|||all u gotta do is buy another

kens5