That use case is much simpler. To "verifiably connect 2 npubs" you should need to sign a note saying that you own the other npub and vice-versa.

If you want that relationship to be done automatically and detectable by clients we can come up with something too, but what would be the use case?

Another thing that can be done is to generate multiple pubkeys based on a single secret key by, for example, adding numbers to it, for example:

pubkey(nsec+1) = npub+pubkey(1)
pubkey(nsec+2) = npub+pubkey(2)
and so on

Such that if someone has npub and npub+pubkey(1) they can verify that they are indeed connected (or it could be any number, doesn't have to be 1, 2, as long as the verifier knows the number).

I think we should have a NIP for this because it's a cool way to have multiple identities that are nonetheless tied together, but I'm still searching for a use case.