Am actually just added a second maroon clown to my tank.
Generally if a clown is left alone (with no other clowns) it will become a female. Two female will never get along and will fight to the death. If you have two males, the strongest male will become a female and the two will likely mate. If they are still juvenals, then hopefully they will become a female and male.
Supposedly the best way to pair clowns sit to add one, make sure it is a female, then add a second, much smaller juvenal, and hope the female accepts it.
It also depends on the species. What type of clowns???? Did you buy them yet, or could the store put them together to see if they mate first???