Ouch -- this is a problem I haven't planned to handle, one that my venerable Mares Abyss second-stages make difficult to handle. The "vortex assist" tube gets in the way of fitting a necklace on the mouthpiece-tube, and when I tried a molded OMS necklace on the mouthpiece, it interferes with my lips' seal (and it's uncomfortable).
If it's a fixed necklace, place it under the cable-tie holding the mouthpiece.
If it's not a fixed necklace you want, just make 2 knots and pull it tight-ish so that the necklace is on the cable-tie holding the mouthpiece.
I don't see how the tube would get in the way? (I don't have such a reg handy, so I'm basing my suggestions on google images.)
As for the whole "long hose, no long hose, 2 long hoses", I dive solo and still have a long hose and my necklace (simply because I never saw any big issue with not having my 2 regs on short hoses). I did consider a setup with 2 long hoses as I'm not comfortable with the idea of handing a reg I'm not breathing (and especially with handing a reg that has 2 spots), but that was when I had buddies with no redundancy.
I didn't put too much thought in such a setup, I couldn't figure a way that would not cross hoses and still wrap around, which I consider the best way as it keeps the hoses away and makes sure there's a decent length available when needed.
If you're into "technical" diving, I don't see a sudden need to be able to hand a reg, we all have at least one other source of air.