Note: This can only be done with a guitar whose pickup covers are not permanently connected to the ground and the minus output.
Doing any unauthorized work to your guitar will void the warranty.
This actually can be done in two ways and the first method involves two SPDT switches to make one series parallel "switch". This method might seem familiar if you have looked at the "Funky Five" or the "Super Seven Switching" and examined the middle two switches of those circuits.
Two SPDT switches allow you to choose:
neck and bridge in parallel
neck and bridge in series
Granted, if you have a two pickup guitar, the odds are good that it already has a pickup switch which can provide the first three options. However, if you want to have both pickups in series, this is one way to do it.
The two switches required for this are SPDT switches that have no center positions. As you know some switches have "center-on" or "center-off" middle positions but they are not necessary for this circuit and it is strongly recommended that you do not use such switches for this project.
Besides the drilling of two holes for the switches, another disadvantage occurs when installing this in a Les Paul style of guitar because the original pickup switch is no longer needed. You might be able to use it as a coil cut switch or for switching different capacitors but the fact that this switch will no longer be needed is something to take into consideration.
Here is the second way to wire a series parallel master switch and it is done with one DPDT switch with no center position.
The advantages for this circuit are:
only one switch, so only one hole has to be drilled
the original pickup switch is used
Unlike the previous circuit, the two switches work in an entirely different manner.
When the DPDT switch is in the parallel position, the original pickup switch functions just as before.
In the series position, both pickups are always in series, and both pickups are always "on" and the guitar pickup switch will have absolutely no effect on the switching.
There you have the 2 ways of wiring a series parallel master switch and it's your decision which circuit to choose.
Good luck with the wiring.