Diving with Independent Doubles

[ianr33]

If you use 1/3rd from one tank, then use 2/3rds from the other tank (you turn the dive once you use the first third but stay on that tank until you finish the second third) it becomes obvious that you still have 1/3rd of the total gas you started with left in the first tank at the turn point. And when half way back out (when you switch back to the first tank) you still have an amount of gas equal to what you need to exit left in the second tank.

2 Gas switches is all you need to ensure an adequate reserve in either tank for you and your buddy.

O.K. but: suppose you are halfway back out,just got down to 1/3 remaining on the tank you are breathing when you lose all gas (2/3 tank) from the other tank. You are now in a situation where you have just enough gas to exit.
That means that there are 2 points in a dive where the risk is greatest. That to me seems to be a needless risk. Switching more often than 1/3rds would mean that any single failure halfway back out would leave plenty of gas,not just enough. Doing a couple more reg switches is a small price to pay for the extra peace of mind.

The above obviously ignores having a buddy around or diving in a High Flow Cave