Splitting humbucking pickup coils with a balance pot?
This depends a bit on where/how you want to insert the potentiometer but generally is a bad idea for passive pickups.
The moment you get more than a few kOhm in series with a pickup, you will damp its resonance a lot making the sound very dull. Single coils are especially susceptible to this.
The same goes for less than about 200 kOhm in parallel to a pickup, which will damp the resonances already significantly.
So if you find a configuration which avoids both of these while blending, it is possible.
For active pickups it is different: you can buffer both single coils, then blend them with a low resistance pot (1-10 kOhm). For basses, this kind of blend knob is indeed widespread, but there it is used to blend between bridge and neck pickups.
Doing it for two coils in a humbucker however might have questionable benefits. After all, the two single coils are so close, that their individual sounds should be extremely similar.