A guitar can have a lot of space when one pickup goes through one effect while the other pickup goes through another. You can fade in effects with the volume controls. Or you can set up the neck and bridge pickups as separate presets and use the pickup switch to bop between them without physically being near a stompox. A Stereo Preamp Cable takes advantage of this. The Preamp Cable circuitry is doubled, once for the tip , once for the ring contact.
Construction is obviously more difficult due to the the physical size issues.
Here in the photo some of the circuitry can be seen iside the epoxy potting. It would be a good idea to hand-match the FETs for the stereo Preamp Cable. A stereo Phantom Power box doubles the circuitry of the mono Phantom Power box. This is what I used on all the 12-string guitar parts on the Tesseract album. Until someone goes into production with it you have to build one yourself.