can u create a sep. gamertag on an existing acct?

just wondering if u can create a seperate gamertag on an already existing acct

my reasoning is my little brother comes and stays with me sometimes, and he likes xbl, so I want to get him his own on my acct so he isn't affecting my score, or unlocking achievements on my gamertag

thanks for any help

Nope, the only options you have are signing him in as a guest. Unless you just don't want him playing on your account. Then you would just have to go and buy another Gold membership. I guess in your situation those options don't sound very appealing.