I have seen, or heard, people successfully keep two different anemones with two species of clowns, but in a 55 gallon tank, two anemones may be a pain in the a**. If you have a fully stocked tank with coral, and either or both anemone decide to take a stroll, you are not going to be happy.
If you do, I would suggest possibly trying to get two anemones where one host clarkii but not percula (bta) and the other host percula but not clarkii (?? actually I don't think there are any naturally). That may help avoid any competition over the anemones.
You could though, try to just add a bta and see if the percula will host it, then if so, get a second bta and have one for each. BUT, sometimes bta do not get along either.
This will definately be the anemone's tank though, so make sure you are able to move coral out of the way when the anemones decide they want to move.
Let me know what you decide.