If there is not a real big size difference it may be the two fighting for domininance. Watch them carefully. If the smaller one is starting to look bad (torn fins or signs of damage, cowering in a corner, starting to show signs of illness) then you should get one out of there. There is often aggression between clowns establishing which will be female. Unfortunatly in a tank that size there is not a lot of space for the smaller to keep out of sight until they work out thier differences.