The
PBG BG-46 diagram appears to have an extra switch on it (a 3rd toggle switch) for the bass pickup selection, which the kit doesn’t actually have. This means it should really be one selector for the guitar pickups with one volume, the bass pickups wired like a Jazz bass with two volumes and no selector, the guitar and bass outputs going to the neck selector and the out from that going to a master tone and then going to the jack.
Do you want to be able to have both necks on at the same time, or just bass or guitar and mute/off in the middle position? If it’s the latter, then
this post has some diagrams that may work for you. If you want the option to have both necks on at the same time, as in bass/both/guitar on the neck selector, then I could sketch up a diagram for you when I get some time in the next few days.