if they are still juvenile and the same size they are probably determining which will be the female (the more aggressive) and which will be male (the least dominant). When this is decided they should stop fighting.
If they are mature, and both male, they will do it until one dominates and becomes female.
If they are mature, and both female, they will do it until one dies. Clownfish can go from male to female, but not in reverse.
How big are they?
"At the age of six I wanted to be a cook. At seven I wanted to be Napoleon. And my ambition has been growing steadily ever since." - Salvador Dali