The best way to get a pair of clowns is to buy them small. The more dominant of the two will become the female. If you buy adult sized clowns then there is the chance that you will have two females and they do not
reverse their sex so you would have fighting going on. Unless of course the store is willing to guarantee
that they are a mated pair!!
Clowns start out life as sub-dominant males. The most aggressive one in a group will become a female and the next in line will become a full male. The rest in the group will stay in that male limbo until one of the pair dies. Then the next in line will move up to become the full male, while the previous full male will then become the female. But they cannot go backwards on this. So two females stay as two females which can turn into a bad situation unless you have a large tank.