It is my understanding that if you want to have more than one you have two choices.
1) Buy a mated pair, the easiest solution
2) Buy one larger one, and one smaller one and hope the smaller one becomes a male and the pair becomes mated.
I, personally, purchased my two percs as a mated pair and they get along great. They chase each other every once in a while, but are never mean or nip at each other.