BIGSAGE136:Think deep & solo. Not only does an H valve give you a redundant 1st stage, it also allows to isolate either one.
And I'm thinking redundant gas beats redundant gas delivery.
I'm no fan of the Pony bottle, but I gotta think the margin of safety on deep and solo is better slapping that second reg on a real pony bottle (not a spare air thingy) than simply slapping it on your main tank.
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Ken