Looking to use some SD P90 rails in a build and came across a vid as part of an Epi upgrade that gave quite a few pickup combos.
4 pot setup with two push/pull volume and three way switch
With both volumes down standard humbucker
Neck volume up single coils
Bridge volume up P90s
Both up humbuckers in parallel
Three way switch acting normally.
Have no idea on how this could be wired : descrition toward end of vid
Seymour Duncan has a diagram for that layout here, one pair of push/pulls controls both pickups.
I went a bit further when I used them and had four push/pulls so each pair could control the pickups separately, and added in some cap switching as well for the neck pup. That allowed me to have an 0.015uF cap on the neck whenever the series humbucker or P90 was selected and 0.022uF for the rail or parallel humbucker. Thread here and diagram: