It is possible. This was explained to me as an acceptable workaround for rulers who want regions but not cities. I'll use Cathay and Ansopen as an example:
I can create a new Duchy out of Taop right now. The current Duchy has Taop and Ansopen. The new Duchy will just be Taop. I can then appoint myself as Duke of whatever the previous Duchy was and I can be Lord of Ansopen, Duke of used-to-be-Taop, and it'll be a one-region, rural (or in this case, woodland?) Duchy.
It requires cooperation from the previous Duke and is a bit of bureaucratic leg work but I guess it gets the job done.