Ok, as promised here's a diagram for the
BG-46 using the stock parts with the neck selector giving Bass/Both/Guitar:
The Bass side is wired like a Jazz Bass, so adjust the bass volumes up/down to get the pickup(s) you want. The Guitar side has the lower pickup selector and a single volume control. The guitar volume is wired like an LP so even when you have both necks on you can zero the volume to mute the entire beast (if you don't want it like that, just swap the blue and purple connections on the lugs of the guitar volume). The Tone works as a master tone for both necks.
For those interested, here's an alternate version using stock parts with neck selector giving Bass/Off/Guitar:
Same basic functionality as above, but the neck selector gives Bass, Guitar or Off/Mute in the middle position. Essentially, the Guitar circuit and the Bass circuit are linked in series across the neck selector toggle switch, and it shorts out whichever you don't want. In the centre position it shorts both circuits giving you an 'Off'. Note: for this layout to work the ground lugs on the Bass volume pots
don't connect to the back of the pot, instead they are grounded as needed by the neck selector setting. So, wire them as drawn if you want the layout to work.
Let me know if you need anything else.