I spent some time last night designing this. It's a diagram for an experiment I am trying out on an acoustic guitar that will take signals from both piezo soundboard pickups and a standard single coil inductor pickup that will be mounted in the sound hole and either have two separate stereo outputs, or blend them as a combined signal.

2 pickups
2 independent volumes
2 independent tones
2 way on/off switch for stereo/mono switching
2 mono output jacks

With the switch in the off position, you will have 2 jacks with separate signals from each pickup.
With the switch in on position, you will have both pickups combined in both out put jacks.

So, here is why! In the studio you can simultaneously record separate tracks from both pickups in the stereo setting enabling you to get more variety and flexibility with one guitar in one take. For example, you could put all the signal from the piezo sound board pickups in the right side of the mix and take the signal from the single coil and put that in the left side of the mix.

In a live performance setting, you can use the combined signals to add a lot of depth and unique tone to your guitar and use the independent volume controls to get just the right blend between the two different sounds getting a nice blend of acoustic and electric tone.

Anyway, I spent some time looking and didn't find a diagram that had mono/stereo switching and so I thought I should share this in case anyone was wanting to do something similar. Any passive pickups should work in this. So if you were wiring a Les Paul for this, you would just replace the single coil with a humbucker and the piezo with another humbucker and you would have basically the same thing, though you may want to ignore the value recommendations at that point.