I've got two BPC's and they do this. From my observation they are not fighting because usually there will be eggs in the tank after their little kissie, liplock dance. The first time I saw them do this, I didn't know what to think. They actually lock lips and rock like a see-saw. Watch for eggs, maybe yours are doing this! The eggs don't hatch, they turn white. I usually vaccuum them up or let them eat the eggs. I don't think you have anything to worry about.