A bta is a bubble tipped anemone. It is not recommended that you keep more than one in your tank unless the first one split to form the second. Often, several different BTAs may bump into each other and "fight."
As for the clownfish, Different species of clowns will often become territorial and fight. While it may never become a problem, people often try to keep a pair on only one type of clownfish to avoid possible confrontations.
Hope this helps,
Mike.