From what I am told, they start out without a sex. When you pair them with another they will determine who is the male and who is the female. The larger of the two being the female. No telling what you have if the sex has even been determined yet. Two females will fight to the death as a rule. A male and a female will get along fine in most situations. Two males are fine together because one will turn female. Males can turn into a female, but females cannot turn into a male. You can get a smaller juvenile and hope that it has not determined or is a male. Then you should be fine. Or find a pair at your LFS and take the smaller of the two if they want to break up the pair and that, if they follow the rules, should determine that it is male.
disclaimer: I am not a marine biologist nor do I claim to be. I just stayed in a Holiday Inn Express last night.
-Az
