Lets run a scenario where we are doing a double stage swim with 104s for backgas. Max allowable gas for penetration is 1/3 of 442 cuft (77+77+288), or 147cuft.
The first stage is dropped at half+200psi (32.5cuft ), same with the 2nd stage (another 32.5cuft). 3600psi/3 = 1200psi. Subtract 100psi per stage, and we now have 1000psi (80cuft) available for penetration. Total usable is 145cuft. Still within thirds, but...
This places your reserve gas in a format that is easily shared (longhose), with you (able to be shut down, isolated, etc) and not turned off, as most failures occur on re-pressurization of the stage reg. If you really need that gas, the last thing you want to deal with us a fubar'd reg.
If diving 1/3 of stages and you do happen to exhaust your backgas when you hit your bottle cache', pick up the stages (one for you one for your buddy), by the time you get back to the 2nd cache', you have 1/3 of a stage bottle locked up in a worthless format. It has to be carried (slowing you down), hose stowed (slowing you down) and is unsharable.
Now, things change in deep cave and when scootering. The best (and proven) solution is to dive stages only and save backgas as pure reserve. This works not only for sharing gas, but for delayed exits (poor vis, light failures, etc). If your buddy has a real bad situation and loses a lot of gas, the answer is to get them on the long hose (NOT stage) and tow out. Otherwise, they'll simply run out of gas again, which will put a temporary halt to the exit and they'll have to get on your longhose anyway.
