I'm not sure there's been any obvious transfers between multi club ownership that weren't fair market value:
https://www.transfermarkt.co.uk/the...ull-salzburg-to-rb-leipzig/index/galerie/2128
Sesko was around 24M euros, which is a fair market value at the time - I don't think he's done enough to say he's worth significantly more than that straight away.
Even Allan Saint-Maximin went for £23m, which was pretty low by most peoples standards.
I think someone will have to take the piss by selling someone way under market value or way over, for the bigger team to benefit, for it to ever be an issue. It is not in their interest to take the piss as FIFA/UEFA could easily turn round and ban multi-club models off the back of it and then they'd be fecked.