Coincidentally that's one of the first things I posted on nostr about :) Yes, 2 people could cooperatively control a nostr npub and use musig2 to sign for it. MuSig2 was designed to do exactly that for secp256k1 keypairs (which nostr as well as bitcoin use). It's a protocol to let N of N individual keys create a combined keypair and make signatures which require the consent of all keyholders. The protocol (musig2 that is) is pretty complicated to make it so that the end product look just like any other signature, and the combined pubkey looks like any other pubkey. HTH.