I believe in futureproofing skills and gear, and as you've mentioned you would like to do tech eventually, you should be aware of, or move towards having a standardized tech setup which normalizes the bungee alternate source and uses the long hose primary for donation. And no other bungeed regulators.
Obviously, do what you want, but my advice is just to start off right, using tech conventions as your guide rather than inventing your own, and starting to think like a technical diver (streamlining, simplicity, redundancy, standardization, sharing gas, potential for hazards like entanglement, breathing the wrong gas). There's a reason for tech gear being relatively standardized, which also includes SPG on hip D-ring, computer on right wrist, no retractors, etc.
Anything that involves an octo around your neck means that you're donating your primary to an OOG diver, so think about the required length and routing. Will it be long enough to reach the other diver safely? Can you both swim with it deployed? Does your existing routing under your arm satisfy that requirement or does it need to be cleared past your arm to reach your buddy?
Two necklaces can be confusing if you need to switch regs (say when donating your primary to an OOG buddy) and put the wrong one in your mouth and get nothing (remember your pony should be turned off unless you need it).
If you need to get the pony necklace over your head, that could flood or knock off your mask. It's just a lot of potential fuss lurking in a real emergency situation.
If you're going deep, you should use a larger stage rigged setup (say AL40 or AL80) clipped to your left front and hip D-rings rather than a smaller pony because you can get familiar with using them for redundancy and extending your bottom times compared to having a small emergency-only pony. You can use the AL40/80 if you do tech, but anything smaller will probably not do.
You can also detach and clip a stage rigged setup to your buddy so you're independent of each other if they run out of gas, and a necklace around your neck makes that hard.
This would be a good time to standardize on regs if you're building a set. All of my regs are Apeks XTX50s second stages with DS4 (stage/deco) and DST (doubles), so servicing is easy, and they are all interchangeable.
AFAIK, PADI standards for Deep don't technically require the student to be the one carrying or deploying the extra bottle, just using an alternate source for one minute during the simulated emergency deco stop. The extra bottle(s) is often carried by the instructor. If this stop is midwater without a line to hang on to, this would be messy with students who have never deployed their own bottles before.
But obviously, it all depends on the site and instructor, and if you have one that will have you run through the deploy and stow process, that is awesome.
