It seems like you would need a third key or a different type that can't sign but is required to piece together all the npubs as being signed by a particular key.
Discussion
This is above my pay grade at the moment though. But it's an interesting concept.
You would share that key privately somehow. Maybe like the in person QR that some messaging apps use.
Wouldn't looking at NIP-17 as inspiration be useful? You could establish the back channel in a similar way to that then update the npub via that back channel.
I'm not sure it's very valuable, but that's how I would begin thinking about it.