If the volume pots for the two coils are in circuit before the selector switch, each of them has the capability to shut off the overall output when both pickups are combined in series.
Take a hint from the Brian May Red Special circuit. One volume pot, one tone pot.
Another thing that I remember from the BMRS circuit is that the most useful series linked single coil combinations for obtaining humbucker-like tones were Bridge+Middle and Middle+Neck. The former is May's main sound. The latter is the chord break sound from "It's A Kind Of Magic".
The one instance where two volume pots makes sense is if the second one acts as a Blend control for the centre pickup.