How to blend two guitar audio signals?
looking at this schematic, this is clearly an active system, it seems as if the preamp circuit is on the PHATII pcb. The circuit appears to have a single preamp, the mix is done by connecting each pickup to it via a 500k variable pot (and also the tone circuit). This is indeed, perhaps not the most flexible way of doing things.
The best way to mod this would be to fit a new preamp module, where each pickup goes first into its own preamp, and then feeds a mix stage via its own volume control. While it is certainly possible to design such a thing, it may well be that this already exists as a module.



simulate this circuit &ndash; Schematic created using CircuitLab
For interest sake, the basics of such a circuit is shown above. A few points:

TL081 probably not a great choice as we will likely be running from a 9V battery. A FET input audio opamp that gets as close as possible to each power rail would be preferable. Also some means of creating a split supply is needed (perhaps a virtual ground IC). The &quot;ground&quot; points shown are actually the mid point of split supplies, of course - &quot;true ground&quot; will actually be the negative terminal of the battery (which the pickup screen connects to).
The input cap with R1 and the pickup impedance itself gives an LF rolloff at about 3Hz. This value (like others) might be tweaked a bit to taste.
R2 is a linear pot (not log law), R6 in combination with R5 gives a quasi-log response (approx 20dB at midpoint). This is preferable as log pots are more susceptible to wear than linear ones.
R7 probably wants a small (say 22pF) bypass cap for stability.
Tone control and master volume control not shown.