There's no guarantees, but if you buy them small enough (~1")they will most likely pair up. There will be some aggression/fighting as these little (unsexed) ones determine which one is going to become dominate (the female). Luckily, percs/ocellaris are the least aggressive clowns, so it should happen relatively quickly. They will then be considered a bonded pair. Eventually they may lay eggs (again, no guarantees) and become a mated pair. That's why I am leery of any vendor calling their pairs mated because they probably haven't spawned in their system, which would just make them bonded.
You could make the pairing a little easier if you get a large and small one. Then, the female has already been determined, and the smaller should submit pretty quickly. You just have to be careful about getting the smaller one to be sure it hasn't changed over yet, so the smaller the better.
I personally would give inland a call since they breed clowns on site and are very knowledgeable.
HTH.