I answered my own question and did a write up here: [Tutorial] One ghost blog, available on clearnet and as onion service
EDIT:
Unfortunately this doesnt work. When I add a new entry on the clearnet page, it is not update on the onion instance.
However when I go to my publisher profile on the onion site, I can see the new entry. Clicking it produces a 404 error.
I have to manually restart the ghost instance, then it works.