The odds of a pair of maroons pairing off in an LFS tank is slim. Your best bet would be to buy 2 or 3 and let them pair off in your tank after a period of quarantine.
I would buy two from the same tank. One should be one of the larger ones in the tank (likely to be female) and one of the smaller ones (likely to be male). If you were to follow Dean's suggestion, I'd get three small and let one of them morph into a female.
Easiest way is to buy an established pair from someone. I happen to have a pair. If you are going to build a pair yourself, put them(One large and one small) in the same tank and in time the larger one will become the female, and they should hang out together most times. Maroons can be nasty fishes, so if you are going to make your own pair, maybe you should put a guppy cage in the tank and leave the smaller one in there. Once they get use to each other, then let the little one out.
I paired up mine with the same advice above. One big, one small. Left the smaller one in a guppy cage for 4 or 5 days. You will see the the larger one being very aggressive towards the little one. The little one will show it's submissiveness by quivering. Even after letting the little one out of the guppy cage, the big one bullied it for several more days. Now they are a happy couple.