sub-zone on the same server but in different backend - how?

[lejeczek]

Hi guys.

To experts that most likely be silly easy but my brain got 
tangled up and cannot get around it now(also being a novice)

Have a zone on a server, say:

- the.zone

with "flat" files being the backend for it. Now wanting to have:

- sub.the.zone

served by the same BIND server, but stored in.. "SQL" backend.

How... well how to make that work if at all possible?
I'd hope it can be done with some "trickery" in config/zone 
files if it is not 'easy-peasy'

many thanks, L.