That depends on how big the tank is and the size/age of the clowns. Clowns sex by age and size until they become females. If both clowns are females, then one is doomed unless the tank is large enough. If they are immature yet, then let them fight it out. One will become female, the othe less aggressive one will be male. If the smaller gets beat up too much, then a divider is in order to let him recover. If the larger, more aggressive one grabs the entire tail and shakes the smaller to pieces, get one of them out quickly or the smaller one is going to be killed. Aggression and shaking is normal between the two at first. The aggression will turn to courting after they decide who will be female.