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I wouldn't be too concerned from that alone. my fish regularly lip lock, but it never leads to anything serious. You should be able to tell of its escalating and you can intervene if necessary.
It could be aggression/territory disputes or they could be testing each other to see if the thee is a suitable mate. I can't tell the sex of them to see if that is a possibility.
I also think its a way they exert and/or test their dominance over the other. My Blood Red Parrot and my Severum do the same thing. I got them both when they were young and they have a love hate thing going. When fins start to get chewed then I would start being concerned.