Hi,
I done exactly the same as you. I signed up for Auctiva today, not realising that the username I picked would also be the name of my Auctiva store. I then registered with a username that I wanted the Auctiva store to be called and cancelled the first account. I had exactly the same problem as you with the second account.
I've just figured out how to solve it and everything seems to be working okay.
What I have done is cancelled the second account and reactivated the first account. I then clicked on the STORE tab, then the Store Window link, then the Edit Store Window link. Under the Store Window Settings where it says Top Text (Title): I changed my username to a name that I wanted to call the Store, then clicked on Save Changes. It took a couple of hours for the change to take effect, but it all seems to be working okay now. I still have to log in with the original username, but the important thing is that the Store name is what I want it to be.
Hope this helps.