BILLB:
Please take another look at H-Valves. I use them on all my tanks. I dive with 120's and with an H-valve, I have fully redundant air! Two regulators (with seperate first stages). Each valve is independent of the other (unlike Y-valves which are not isolated).
You do not have totally redundant air, you have a redundant valve and redundant regs. If you were to have a valve neck leak you would not be able to isolate.
Although a tank neck leak is uncommon, to say you are totally redundant is incorrect. Two tanks with seperate regs joined by a manifold is redundant, as is independent doubles or a pony.
Lets take this a bit farther. Supposed your pony bottle had a cracked neck? (I have observed several pony bottles with cracks).
What if one of your doubles had a cracked neck or slow leak? Would you continue to dive? My terminology was not entirely correct (thanks for the correction). But the point made was that if you are uncomfortable or have no experience with doubles, H-valves on single tanks offer a next level degree of safety (fully redundant regulators/back up gauges/computers, Etc.) Diving with a pony tank plays havoc with trim unless you sling it in front but the extra drag is still there. And, to do the depths that NJ diving attractions require, a pony would need to be 30 Cu Ft or more to do a "safe" ascent (and probably no safety stop).
Obviously, diving doubles is the ideal. No argument here.
