Having two noble-lead units fight with each other would be intra-realm warfare. I like the idea, but it has been rejected before.
If this is region based, maybe it would make more sense to base it on militia. Make the militia set to murderous when they see a particular noble or something. It should always be possible to go through a militia-less region.
Intra-realm warfare has been rejected because they always suggest duchy warfare which takes away from feeling like a team but one noble fighting another noble is basically a duel but with your unit so it isn't the same type of intra-realm warfare that has always been rejected.
