How to wire 2 pickups permanently in parallel
 

I have 2 jazzes - One with a parallel/series switch and one that is permanently in series to act as a plug-and-play backup to the other. However my primary bass is now being used mostly in parallel and thus i want the backup to be wired in parallel for now.

How do i wire the two single coils in parallel into a single volume pot?

Wire it like a P bass, but with the coils in parallel.

So each pickup has a hot and ground wire, where do i wire each of these 4?

Two wires have to be soldered to GND, two wires to HOT ...
That's the parallel HB wiring ...

The wires which are soldered to GND or HOT stay there.

The only question which of the other two (at the moment connected) wires belong to which coil. You can test this with a multimeter or make it my try-and-error. There are only two possibilities which wire has to be soldered to GND (or HOT).