A lot of cichlids lip lock, even discus and angelfish do it. Basically they go mouth to mouth and push each other around. If both partners are similar in strength they might become a pair. If one fish is noticeably weaker than the other, they don't normally become a pair.
When 2 male fish lip lock, it is part of the territorial display. They start out by flaring fins and circling each other. This is a bit like a couple of guys circling and eyeing each other off before they get into a punch up. If the fish are similar in size and neither wants to back down after showing off their fins, they get into it and grab each other by the mouth and push each other around. This will continue until one fish submits and swims away.