Adding a dummy coil to another pickup is usually done to make the working single coil pickup humbucking or quieter in the instrument. Some companies use the pickup directly under the strings without using pole pieces and others rout a cavity away from the proximity of the strings.

Using a dummy coil the DC resistance will change along with the tone and frequency response of the existing pickup. The inductance will change when testing a pickup. The dummy coil can either be used in series in phase or series out of phase, or parallel in phase or out of phase. When adding coils remember it can change the tone or phasing of your instrument when used in combination with other pickups.