It's not complicated, but the tricky part is finding a dual gang pot with a centre-detent, so that you'll feel when the balance control is at 50/50.
Blade used t make their basses like this and it made a lot of sense.
If you use a series/parallel switch it will alter the sound of the bass, but in series mode you won't be able to alter the blend of the pickups - they're on equal output all of the time.
Better call Ki0gon