Actually, clowns are born neuter and as the "pecking order" is established, the largest will become female and the other will become male.
In the wild, some species (ocellearis included) congregate in groups. The largest will again be the female, the next largest will be the male and the rest will remain neuter as long as they stay within that particular social group. When the female dies, the male will move up to become the new female and the largest of the neuters will then become the new male.
With most species of clown, if you can still buy them while quite small, there is a much better chance they are still sexually immature and there will be a much greater chance of bonding and less of a chance they will kill each other. If there is already an established clown in the tank, adding a possible mate becomes much more difficult and is best to ensure the new arrival is less than ˝ the size of the current clown. This will help ensure the new clown is not seen as a threat and will not already be a female.
When purchasing the ocellaris you want, I would look for TR
if possible and as close to 1" in size. That should greatly improve your success.