But after the plane has returned to the stand, the question that now arises is who do you offload? One, both or all?
IMHO it always takes two people to start a fight. Even if there is provocation from the other party, you always have the option of ignoring it and postponing any further discussion until you're no longer on duty. Once the fighting has started the Captain should have all individuals off-loaded as clearly unfit for duty and let HR sort it out on the ground, as it will then no longer be the Captain's responsibility (and it shouldn't be).