I asked a similar question a little while ago and this response from jwtrojan44 answers it well I think. I have one Oceallaris clown now and am planning to add a second.
"A single clown is fine by itself, but there are few sights more associated with sw tanks than a pair of clowns together. You could certainly add a second but be aware that there may well be some aggression as they establish sexual dominance. If the present clown is small, try adding one of the same size. As they have not reached sexual maturity yet, they should pair up eventually, with the dominant one becoming the female. If the present clown is larger and more mature, add one smaller but again, be watchful so that it does not get too physical. Shimmying, shaking, lip locking and tail fanning are common rituals. Normally the aggression, if present, lasts only a few days. I have a pair in one of my tanks with the female being at least twice the size of the male, and all is bliss. Good luck."
