Two accounts, one authentication key

My brother recently wanted to get an account that he would have his own characters on and pay for it himself. I already have an account and bought both world of warcraft and the burning crusade. He has a 14 day free trial membership and a 2 month pre-paided card ready but he can not create an account b/c the authentication key on my discs has already been used (by me). He would be paying the 15 dollars a month and I would too but we really don't want to have to buy both world of warcraft and the burning crusade again just to get a new authentication key for brothers to play in the same house.

If there is any way to reasonable way to fix this, please help.

Thanks.