If they are medium sized it may be difficult at best to get them to pair up, especially if they have been alone for a while. Once they start female they can't go back so they will never pair up and may fight constantly tank size, space dependent. If they are both the same size and not from an established pecking order they will fight until dominance shows up. Two small fish will fight but both will be gender free so dominace will eventual win over. Twice now I have purchased a small fish to match a large that were a few months apart but the larger was more submissive then the smaller and they continuously fought never pairing up over three and four months. He/she is now significantly larger so I will be getting a tiny clown in a month or so hopefully with better results than the last two pair attempts. Note, I just have a dumb fish not the standard!
Just like nature some fish have a "not if you were the last clown in the ocean" mentality, but most will do alright when not given any other choice in their relationship.
Best way is to purchase one large and one small, if you can't only buy one that is large, then wait until you see a large group and take a smaller one. If you don't know the sizes of the ones you want to buy only get one. You can always have it alone for a year or so if need be then get another smaller one.
I wonder if tank raised leads to psychological disorders in fish. My Oscellaris won't even go near an anemone or show any signs of hosting other than the overflow wall, lots of LPS and BTA. Most captured fish host right away IME??? not concerned about not hosting the BTA but most would host a frogspawn or flowerpot.
