One more thing: you rely on A and B not keeping and later combining those private key parts.
You're right. I didn't mention it because I thought it's obvious, and unavoidable. Funded collectibles are just not compatible with "verify, don't trust".
That would require destroying the hologram?
Or the designer keeping the partials.
----
vs
I hope they nuked these keys.
It's not the ultimate solution but it's enough to make the effort redundant in the long run.
It's yall's call. get compromised 100's of wallets at a time, or detect foul play 1 coin at a time from "potential" bad designers.