The distance a pickup is from each other can vary on the design of the instrument and tonal variations the designer is trying to accomplish. I like using just one pickup and I can get many variations of tones out of it. Some builders claim they have their pickups mounted at the precise harmonic of the instrument. I think they forget that once you play on the first fret, the harmonic isn’t over the pickup anymore. I believe having the pickups too close can change the magnetic field to the strings. If the pickups are too close, the magnetic calibration can change affecting the tone and output of a pickup.