That really changes things in terms of privacy.

The events wouldn't be visible to other npubs or relays & so couldn't also be decrypted following an nsec leak. The existence of the events wouldn't even be visible.

The big drawback would be the need for both clients to function in this way & be reachable.

I suspect that a better compromise might be using a specified relay for exchanges with certain npubs. A trusted local relay for certain events.

This might then pave the way for semi private (non encrypted) event exchanges using agreed relays.