Ascending Double Rope-Petzl Shunt

https://m.youtube.com/watch?v=WsaSNUMCm44

So this is the most sophisticated way I know of to ascend double ropes, but I was wondering if the VT prusik (which I've only ever used in the tree industry) in this setup could be replaced with a Petzl Shunt. The Shunt looks like a convient back-up to for top down route development, but am also curious about it's other uses. It looks like a versatile and handy (if not expensive and bulky) device, but it's hard to find anything on it that doesn't relate to TR soloing, which I don't do.

I use a shunt whenever I am rapping in from the top of a potential crag. You can easily ascend short distances with just a shunt and ATC (like to get back up if you discover a car size death block on your rap line). If I were intentionally ascending a double rap line I would add foot loop(s). I have been using for approx 10years and am happy to take a stab at any questions you have.

The thing I like about it is that the lever (cam in traditional ascended) that pinch-grabs the rope is totally smooth and gentle on your rope.

After watching that video that is not how I do it with a shunt. So to begin with I have the shunt rigged to my belay loop above my ATC and extended with a standard extendable sling. So both devices are connected to my harness.

Hang on shunt. Use left hand on the locking biner attached to the lever of Shunt and feet on wall (or add leg loop to shunt if free-hanging) to thrust your body and hips up while pulling upward on the break strands coming out of the ATC to pull through as much slack as possible through ATC then lock off break strand. Hang on ATC and slide shunt up rope until it’s tether comes tight. Repeat.

What makes this extra easy is that you do not need to change your configuration at all when transitioning from rapping to ascending and back.

This is the same movement you use to release the Shunt after hanging on it (to finish your rap after stopping and hanging)  
 You can also just squeeze the shunt open and “drop” onto your ATC, which can be easier if the shunt was only partially weighted, but near impossible if you are fully hanging on it. 

Thanks Nick. I was envisioning attaching the Shunt to my harness via my PAS so your set up sounds exactly like what I had in mind. This eliminates the need to put the ATC in guide mode, correct?

I also assumed this would be placed below the rap device while descending as are most prusiks, but you instead run it above it. I've heard that running a shunt above your rap device might cause it to not lock should you have weight on the rap device. Have you found any truth to this assessment? I had a concern that while bolting it might be possible to accidentally push the lever on the shunt and release the lockup with your body were it attached directly at the belay loop, but extending it solves that problem.

So I have it above always. Pinch it open with left hand when rappelling then let go when I want to hang. One I finish fine tuning if my positioning I usually tie off below my ATC just in case. The transition from hanging on the shunt to not and vice versus is pretty sharp. Unless you just barely let it go tight before locking ATC off (for example navigating over roof) in which case you can squeeze the shunt and get it to release. it would be very hard to release it manually if you are hanging full body weight on it.

also I have not tried but I think it would have a good chance of catching you even if you rapped off the end of your rope with the ATC. It really catches quickly when loaded hard.