Thanks but that's basically just searching on two tags (after assigning everything in notebook x to a tag)... my use case is a single search, that I can put in shortcuts that would combine:
notebook:x ... let's say this has 1 note
notebook:y ... has a note with tag:z
The search:
any: notebook:x tag:z
which returns the notes in notebook x and all the notes in notebook:y or any other notebook, that has tag:z