I dont think that is quite correct unc, the BT account is also a yahoo account using the same system and that is why normally you cant login to two at the same time.
What you can do is open two dufferent browsers and loging to the different accounts.
I did see an hack the other day that I think was about letting you login to messenger with two accounts at the same time.
By the way when I last counted a few years ago now I had 600+ yahoo accounts, dont ask why.