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Envoy II 2nd does not
Onyx II 1st is environmentally sealed, while the Envoy II is not.
the breathing adjustment knob on the 2nd were the main factors that had me pick the Onyx over the Envoy when buying for my youngest daughter.
I see no problem with that. They are similar enough, and you can keep the Envoy 2nd in Pre-dive to prevent freeflows as it does have a Venturi switch.What about buying an ONYX second stage by itself and using it for primary second and relegating the ENVOY second stage that came with the ENVOY first stage to be the alternate?
(I ask because I am evaluating which one to use for the dive school).