I am not actually sure what you are trying to do...
re multi select...
I use quite a lot SELCET DYNAMIC, and have that set to multi, I also apply chosen.js to it.
In my db, I store values i.e. "123,234,345,456"
In search I apply 3 Match as any words exact, multiple to allow user to search with more than one value... stuff like that.
Field x is really for when you want to create new values in db as opposed to referencing data that already exists (because if data exists in db already can just use multi select)...
Re fieldx, you said it's storage was none.... o that is why I suggested reversing that