rstark once bubbled...
Would A Y-valve be a better option?
Compared to an H, a Y more closely approximates doubles in that both knobs are in the same position on either side. They are also not on the centerline.
Better? You have the redundancy of the 2nd 1st stage, but still have no backup for a lost tank neck O-ring nor backup for simply going OOA due to poor planning.
The tank o-ring failure is highly unlikely, and poor planning can just as easily result in an OOA with a pony or doubles. More often than not, the use of a pony results in "oh gee, I have another 40cf hanging here... that's half an 80, I can run my back gas to 100psi".
Doubles? Back gas is back gas and poor planning will run them dry as easily as a single... problem is that running them dry is more critical because you're probably now into significant deco.
I'd say that if you can handle operating BOTH valves, and can deal with reconfiguring when you go on vacation, then the Y would be an improvement over a conventional single setup.
If you can't EASILY operate both valves, then I'd say stick with a single. Reason I say "easily" is that if it's not easy, you aren't going to get into the habit of checking them during the dive to ensure that they have not rolled off. It's no fun donating your primary and then discovering than your backup is dry because the left post has rolled off and the knob is broken off. Regularly check it.
