It would be allowable for two Kenoma to fight, though the resulting consequences will depend on which is the one getting serious hurt or dissipated. If the one one trying to help the Pleroma was destroyed, the person who destroyed them would be commended and the destroyed one would have their Shard hauled back to Achamoth. If the one trying to punish the 'betrayer' was destroyed, Xishen will either end up destroying or binding the person that did it.

Essentially, Kenoma won't be punished for loyalty, even if that loyalty comes in the form of attacking another Kenoma. They will get punished for directly obeying orders. In the context where this rebellion is severe enough to get someone destroyed, the punishment would be a path straight to the Regent's prison with a follow up of brainwashing. In practice, this would result in them being mostly out of normal play until the next event, and missing some aspects of the Kenoma's introduction to Achamoth. They would generally have what they need to move on as part of that log, though.