Spectre
Contributor
H2Andy:
Go with progression (or perhaps evolution).
The monkeys:
Singles give you no inherent redundancy. If you have any catastrophic failure [uncontrollable regulator freeflow, hose burst, burst disk failure, neck o-ring failure, neck failure, valve failure, etc.] your gas will be gone or unavailable to both you and your buddy, depending on the type of failure.
Neanderthal:
Singles with an H valve gives you redundancy to a failure from the first stage down through the second stage, allowing you to shut down the valve that the regulator is on, saving the remaining gas for you to use. The cost is that you've added another valve that can fail and result in all your gas to become unavailable to you and your buddy. Also there is a bigger bouyancy swing from full to empty.
Cro-magnon:
Doubles with a crossover valve adds no inherent redundancy other than to provide you with more gas, which results in more remaining gas should a failure that singles with an H valve gives you redundancy for occur. You also have more gas, therefore more time using the gas before it's drained to get to a gas donation. Since most situations allow you to have an amount under 1/2 of your gas remaining after a failure [depending on isolation speed], the bouyancy swing is less. The added cost is weight, drag and expense as well as adding failure points in the crossbar connections.
Fully Evolved:
Doubles with an isolator gives you useable gas remaining should most any catastropic failure occur, with the exception of a failure of the added isolation valve; as opposed to any failure from the first stage down. In other words, H valves and crossover bars give you insurance for any failure from the 1st stage down. Isolator gives you insurance for any failure barring the isolator valve itself. The added cost is the 3rd valve failure point, as well as the procedural issues coming from potential dangers of unknown gas contents [should one tank have different contents than the other and you isolate the mixing of the two. sidenote... always isolate and analyse both posts should you ever suspect your isolator was closed while filling].
Independant doubles/sidemount removes the isolator valve failure point at the added cost of a second high pressure hose/gauge. It removes the time pressure to isolate in the event of a failure. It removes the risk of gas contents. Sidemount changes your profile to be lower and wider than backmounted doubles, allowing for a wider range of passage access. The added cost is that should you have a failure from the first stage down, the reserved gas in the failed side is unusable. It also adds procedural issues of even consumption between the tanks.
My summary:
For all technical diving situations, I personally feel that nothing but the highest evolution is sufficent. That evolution being to either independant or manifolded doubles [where the risk vs. benefits between the two are debatable as to which is better]. Some people believe that the procedural issues of filling isolator doubles is a risk that outweighs the risks of stepping back to crossbar doubles... although I don't agree with that.
