I had a similar situation when servers merged (from saronia to Sargatanas) one was my original conjurer (made at launch) and my main tjat I had played since my return in feb til now. I wanted to move them both to the same server but keep character #2 as little pet.
It was just a matter of logging on to the primary character before logging on to the secondary one. It will give you a prompt that the character name is already taken, and to change it, while still on the log in screen.
At least, thats how it was with server merge ^.^
I would assume (and hope for you) that it's the same ^.^

I hope that helps!
Good luck!