Account changing into my name

This is two questions in one. This account is under my husbands name because I didnt know you could have 2 accounts under the same name. I wanted to be able to:

A. change his account name to mine so i could move a character onto his account....or is there a way to do it since were the same address/same credit card? I wanted both accounts in my name anyway but didnt think you could have 2 in your name, so i put it in his?

B. said character is a DK, can a realm have more then one DK, IF you pay to realm change it on? This character im posting on is HIS main, and he wont let me delete it, but i wanted to move my dk onto this account since my slots are full on my account, and I use it for professions.

And also, would an authenticator then work on both accounts, if they were both in my name? Or would I have to buy 2?