Wacky guitar idea / pickup sensing area

I had a very oddball idea for a 12 string guitar with a separate output for the octave strings that is wired into a on-board pitch-shifter that would allow you to change the harmony of the strings from octave to fifths etc.

I know this idea is probably unfeasible but I want confirmation before I scrap it.

Is it possible to have a single string pickup with a small enough sensing area to not pickup the adjacent octave string?Does anyone make something like this? Do I need to wind it myself? Is it possible to make a pickup with this small of a sensing area?

I know that there are more practical ways to achieve this effect with harmony pedals but I was planning to use this idea on a double neck guitar so there would already be a 12 string neck already. Also it is cooler this way.