Bangs right
if you don't put them in at the same time and let them astablish there teritories, then you shouldn't put another one in with the one you have already. yours has already astablished that the tank is hers, if you add another one, it will most likely be killed. sorry. but if you can catch the one that is in your tank now and put it in a quarintine or holding tank for about a week, then get a new one, you can acclimate them both into your display at the same time, you will have a better chance at pairing them.
i only know this by experiance, this is what i had to do.
and i don't know if it will work for you or not. but i do wish you luck. and maybe there are other people out there that can give you some more info on this matter than i can.
tonya