Welcome to RDO!!!!!!!!!
It IS possible to keep several anemones in a tank, HOWEVER, it may not be the BEST idea ever. I will provide some of what has been going on in my tank, even though my "issue" only started 2 days ago.
I inherited a large sebae (base is roughly 7 or 8 inches across) 2 days ago, so it is rather early to tell how things are going to go. I have a 30 gal on standby in case things don't go so well.
I already had a carpet in the center of the tank. The carpet seems to care less whether its there or not (it's hosting a percula and oscellarus). The problem with the sebae is its size...it basically bulldozes its way around the tank and has jammed itself in a powerhead (since relocated) twice. It finally seems to have settled down now, a little too close to the carpet (for my liking) and had to move one of the rocks to place a barrier between two different species of clownfish (the sebae is hosting 2 tomatoes).
In the long run the 2 may decide to "invade" each other's space or they may not. I simply cant tell at the moment.
After saying all of that, this may go well, which is what I am hoping for.
The best thing you can do is find out what anemone you want to put in the tank, research the stuffing out of it, ask some more questions and then ultimately, you are going to have to decide whether to go for it or not. It could be a pleasant experience or a not so pleasant experience, yet either way you will learn SOMETHING out of it.
If you decide you HAVE to have another anemone in the tank, I would suggest it would probably be best to get a small one (this is based on my experience with the sebae's rumbling around the tank and upseting most of the tanks inhabitants). This will allow the new one to adjust to the tank without beating the crud out of the tank and the BTA you already have.
I believe that there is an article or 2 in the Advanced Aquarist magazine, here on RDO. It would be good to check it out, even if the article isn't there.