Be very very careful when buying 2. If you get 2 males, they will fight to the death very quickly. Your best bet is to get the two that look the most different. It is said that the males have a larger jaw since they are mouth brooders.
As always when purchasing thses fish be sure to see them eat and check carefully for any injury or discolleration. Starting with a healthy fish is step one. Step 2 is to keep the tank COMPLETELY covered at all times.