Ideally you should get either two small ones or one small and the other medium. If both are medium, it's possible they might have both morphed into females already. Two females in the same small tank would not do well together.
FYI, all clowns are born male. As they age a bit, the largest, most dominant one will morph into a female. If that female doesn't survive or is removed from the tank, the next largest most dominant male will morph into the female. Two females in the tank will fight.