They are actually one of the easiest fish to identify by sex. Like the other two have said, they either have a spike (and are therefore male) or they don't. I'm surprised that you didn't know that until now, d34532, it would just take a two second conversation with any mildly competent LFS employee to find that out. (Sorry for how that must sound but if you were interested in pairing up the mandarins, why wouldn't you just ask someone that question before you tried it? Of course, if the people at your LFS really didn't know that, then I apologize, and I personally wouldn't take their advice on too many subjects in the future, as that is one of the most commonly known facts in the saltwater fishkeeping trade).
Anyway, my point is that there doesn't have to be any experimentation when pairing these fish (unlike so many other species), you can usually spot the males from the females a mile away, figuratively speaking.
But I don't know if 2 males of the same species (there are 2 mandarin species that I am aware of) will coexist at all, even in that sized tank. Maybe someone else knows exactly how that works. I've always been told that it is a major no-no, and that they will fight to the death. But perhaps space settles all conflicts in reefkeeping, and I know that 210 gallons should be able to meet the needs of 2 mandarins. It's just the matter of mandarin territoriallity I'm not sure about.
Of course, that's assuming they are both males. If you find that one is a male and one is a female, they should get along fine. If that is the case, there is also probably a good chance that they will spawn in the future.