OP
SuPrBuGmAn
Contributor
If you dive the setup properly, you'll always have enough gas to exit out of either tank independantly. In cave diving, we'll dive 1/3rd out(max) of each tank and then turn our dive, leaving 2/3rds remaining gas in either tank. If we lose a tank at turn pressure and max penetration, we'll have 2/3rds left in the opposing tank to exit safely with. This gives you true independants and safety as far as gas goes. This goes for independant backmounted doubles as well.
An isolated set of doubles has the additional failure points of the isolator valve and cross bar. If you have a catastrophic gas loss in manifolded doubles, it may take you a little while to affectively isolate and shut down the correct tank. This could potentially lose you more gas than you would have lost from a single tank.
A sidemount setup has the valves sitting in an accessible area, so if there's a catastrophic loss, there's no question on where its coming from, and your gas loss will be limitted to just one tank(which won't be catastrophic to your life).
In the highly unlikely event where you lose the gas out of one entire tank, and the regulator malfunctions on your remaining tank, with the sidemount tanks in an accessible location, you can swap regulators underwater.
IMO, sidemounting tanks are much safer as they are truly independant. That being said, you have to stick to your gas planning(which should be done with any tank configuration). Losing an entire tank will end a dive, but you should exit safely without any added confusion associated with locating a failure and isolating it.
... and they save the hell out of your back.
