There are many ways you could wire them up. Series, parallel, splitting, out of phase, half out of phase, etc. You name it.
The Axis Super Sport / HH AL style wiring scheme is pretty versatile. The 25th wiring is neat as well, though it's more involved. An email to CS would get you either schematic, then it's just a matter of ordering the switches (you can get all the parts from EBMM) and you're off to the races.
I couldn't find anything at Seymour Duncan that fits the bill for 2 Humbuckers, 1 vol, 1 tone, 3-wa switch and then adding a series/parallel switch......
Dunno what SD has at their site. I do know EBMM can email you schematics, so if you want to wire it up like the Axis SSTP, 25th, etc. email CS. But both of those have 5 way toggles, not 3.
If you want to use a 3 way and an extra toggle for the series/split/parallel switch, you can do it like this (except with one 4PDT on/on/on switch rather than 2 x DPDTs).
Each coil of each hum on the sub are brought out. They're twisted and hanging next to the switch in stock form to make the hums series. It's just a matter of deciding what you want to do and wiring in proper switches. I would say that you want to be sure that the resulting switch arrangement needs to be something that you can remember when playing live (if you're doing that). I went way overboard with my Sublhouette, getting 6 switching options for each pickup. I opted to be able to choose either coil separately rather than run parallel, but certainly a series/parallel choice could be made. I'm able to switch my sub to whatever I want because I did make the switches somewhat logical (to me)....hum, single-screws, single-magnets, then reverse phase of each.... In the end, I feel my Axis with simple coil splitting does everything I need and will be swapping out and going to an HSS setup with a simple coil split sometime soon.
Thanks. I want to keep the stock pickups as they sound great to me. I think figuring out a simple way to switch them between series/parallel with an additional switch or push/pull tone pot is all the fancy I want.
Easiest way to work them then would to to wire in a single 4PDT switch for series/parallel (forget split for simplicity). A bit of wiring, but a single toggle plus your existing three way.
With push/pull pots you'd need one for each HB. You have a bit more control, but it's more difficult to work on the fly ... I'm not a fan of tugging on a pot before a solo, etc.
Don't be afraid to do push-pulls. The one I got for my Axis was an easy fit. I set them up to be coil split when I pull.....hum push. I tend to want the hum for a solo, so it's easy.