Ill give you my line of reasoning: I back mount the pony bottle, its a 40cf (so it can be used as a stage bottle too in other diving and it has plenty of capacity) its on my left with the reg on a shorter hose coming over my left shoulder onto the necklace (which I did figure out how to make BTW). I have my main tank and its reg and octo coming over on the right. This works for me in the following scenarios:
- Diver is OOA/Paniced/Confused and grabs my primary, since that hose is longer the panicked diver is away from me (granted closer than with a separate hand off pony). I can either look down to the necklace and use the pony or go for my octo. This gives me a benefit over handing off the pony, it preserves my access to two independent air sources.
- Solo diving my primary reg/tank/whatever fails: I have super easy access to the pony bottle's reg and air. A necklace is easier than undoing the bungee on a sling...IMHO
- I am stupid and run OOA myself: easy to get to pony reg
- In a highly unlikely and probably deadly entanglement situation: it is easier to get to the necklace than to a reg that is mounted further away....
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I agree with your reasoning.