Hi all, I . . . am entertaining the idea of putting my primary stage on a necklace.
I will echo the comments that several have made. Do not put your 'primary' - that is, the second stage from which you primarily breath during a dive - on a necklace. Rather, put your 'alternate' (better term than 'octo') on a necklace, A few caveats come with this recommendation.
The value of the necklace is most evident at the time that you donate the regulator that is in your mouth (primary) to a diver who is OOA. Then, all you have to do is put your alternate second stage - located immediately under your chin on the necklace - in your mouth and resume normal breathing, while helping the OOA diver calm down, and begin a normal ascent with you assisting as needed. Some divers even configure the necklace size so that they can put their alternate in their mouth without using their hands (as simonru mentioned).
You want to put the second stage with the
shorter hose on the necklace. For many / most 'standard' regulator set-ups, that happens to be considered the primary, and a common (although not universal) 'primary' hose length is 28-30".
You want to breath from, as your 'primary', the second stage on the
longer hose. For many / most 'standard' regulator set-ups, that happens to be considered the alternate second stage, and a common (although not universal) 'alternate' hose length is 40".
You are best served having two identical, or at least equivalent (in performance), second stages. That eliminates the (unfortunate and unnecessary)
performance distinction between 'primary' and 'alternate', and properly emphasizes instead the
functional distinction between the two - the 'primary' is what you normally breath from, and what you donate in an OOA, the 'alternate' is what you use as a back-up after you have donated your primary (or if you have a problem with your primary). They serve different functions, but they are equivalent in breathing performance.
In your post, you indicate that you are looking for a way to secure your primary second under certain conditions - e.g. a surf entry. Putting the primary on a necklace is not the best solution. Rather, putting a bolt snap on the primary hose (I put mine at the junction between the hose and the second stage), and clipping that primary / hose to your BCD, while breathing from your necklace alternate, during entry is a better solution, as several have suggested..