Series vs parallel for microphones (electric guitars)
If this is two separate guitars, pretty much your only option is a parallel connection  That's because these are unbalanced, hi-impedance signals.  If you were in fact to wire these in series, touching any of the metal associated with the top guitar's wiring(metal connector shell, etc) would cause hum and other noises.

Pretty much any of the passive audio mixers that I've seen just connect the hot signal from each guitar to the top end of a potentiometer with all of the ground connections connected together and to the bottom end of each pot.  The wiper connections of each pot go to a summing resistor (about 100k for guitar signals) and then to the output.  Pot values for a passive guitar mixer would be somewhere between 100k to 1 Megohm, audio taper.

A passive mixer such as I have described has a lot of flaws but they do work. And they will allow you to adjust the level from each guitar individually.