It's too early for my brain to properly function, the coffee hasn't kicked in yet and I've got to get out the door and head to work but... I'm guessing it has something to do with the rod piezo being made up of 6 small individual piezos connected in series. So when you add the disk you have a mixed parallel/series network. Why this results in low output of the rod I don't have time to figure out right now... but maybe knowing this will send you down the right path for research.
The piezo elements used in the Rod type piezo is of a different type. They have less output and less sensitivity than the larger round piezo type.
sounds like the disk is shorting the rod. If you wire a 100 ohm resister in parallel with a 1 ohm resister, you get a result of 0.99 ohms. perhaps wiring them in series instead?
If you connect the rod and disk in parallel, any signal created by one travels not only down to the volume pot but also backwards up the other's wire.
think of one as a 12v battery and the other as a 6v battery.
Could be the polarity of the disk doesn't match the wire colors? Try switching them.
Rod Piezo's have less output than the round disc type which is why most people use a preamp with the rod piezo. Rod piezo's have less handling noise and less attitude that gives them better acoustic tone.
You could put the round disc piezo in the neck area of your git and the rod piezo in the bridge to give them a more equal performance. You could use a on/on/on 3-way switch to use either or both in parallel. You may also have a preamp bypass lug on your jack to wire up the disc piezo?
partly because the vibrations at the neck would be less than the bridge but mainly because these rods are supposed to be under compression, directly under the bridge piece.
Correct. The string vibration will be stronger at the bridge area and the rod needs to be under string pressure at the bridge.
So you'll be lessening the output for the disc at the neck area and giving the rod piezo the most exposure to try and equal out the output.
Disc piezo at the neck and body junction will have a more mellow/warm tone. Upper low string side of the neck for more bass, lower high string side for more treble.