The PRS piezo preamp allows you to run the piezo either:
1) Out of the magnetic output jack as a pre-blended signal with a single standard TS instrument cable,
or
2) Out of the discreet Piezo guitar output jack. When you use this jack, you can route your magnetic and piezo signals independently to different sources using two separate standard TS instrument cables. This is by far the best sounding way to use the PRS piezo.
I thought you might have preferred to use a single stereo jack with a single TRS instrument cable, which you could do after modifying the internal guitar output wiring. The only problem with that method is you must use a TRS cable into a Y splitter on the ground, or you will be forced to sacrifice whatever output you have soldered to the "ring" lug of the stereo jack.