It's one thing to talk about doing this in theory, reality is something else.
The intent is not to pick on anyone but rather to evaluate what was done, why, and wether there were better options by looking at the extant examples:
In your example, the diver had two deco bottles and had a failure on one of them, using a single deco reg to access the contents of both deco bottles. I am assuming that he had succesfully used the first bottle and was switching to the second deco gas. In this case, had the swap failed, he still had his back gas reserve and could fall back on a lost gas contingency plan to safely end the dive.
This is an important point to make as if it was the first deco gas and the swap failed, the odds are good that his reserve back gas would not have been adequate to allow the much longer deco on back gas alone if the dive were deep enough and long enough to truly warrant two deco gasses.
In this case, the diver would have to rely on the deco gas reserves of other team members and may now be doing this in a situation where the team is now outside the original plan due to the effort to switch regs (in the case of the switch occurring with a low percentage deco gas at depth).
That's a very poor position to put the team in. Consequently, I might consider a reg swap but only if it can be done without risking both deco gasses, and only if it can be done after transitioning to a lost gas scenario and does not further delay the dive plan and team. In the event of a reg failure on the first deco gas, that criteria will not be met, unless it's a five bottle dive with an empty bottom mix stage that can donate a reg.
It's also not stated but it is a last ditch "the gas planning wet seriously wrong due to serious dealys in the exit, and I've also had a reg failure" scenario. One of the positives of sidemount diving is that you are in essence diving independent doubles and, properly managed, there is enough gas in either tank to get you out of the cave or at a minimum back to your first deco gas, stage, etc so it virtually eliminates the possibility of total gas loss.
The qualifier here is that sidemount entries usually result in low viz or no viz exits that are much slower, so "thirds" gas management is not enough in terms of conservatism. In other words, a low viz exit is not an "emergency" but rather the norm, and one that should not require you to use your reserve gas. Consequently, if a reg failure poses a problem, the problem is not the reg failure but rather a lack of sufficiently conservative gas planning.
Planning to swap your sole remaining sidemount reg from one tank to the other in anything otther than a dire last ditch situation, would again just be adding one error in judgment/planning on top of others.
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In either case you need to think long and hard about the logic of switching a reg and you need to consider the systemic impact of the attempt on the dive and on the team, as well as on other options that while they might lengthen the deco, are much more of a sure thing and represent a far better contingency plan.
The intent is not to pick on anyone but rather to evaluate what was done, why, and wether there were better options by looking at the extant examples:
I know a diver who was on a dive with someone who tried this with their first deco gas at 130' bottle. It worked, but it took a lot longer than planned with the result that one of the divers on the team surfaced with only 300 psi. The take away from that was that it is an action that once started may quickly place you in a position where you need to complete it to safely end the dive.
In your example, the diver had two deco bottles and had a failure on one of them, using a single deco reg to access the contents of both deco bottles. I am assuming that he had succesfully used the first bottle and was switching to the second deco gas. In this case, had the swap failed, he still had his back gas reserve and could fall back on a lost gas contingency plan to safely end the dive.
This is an important point to make as if it was the first deco gas and the swap failed, the odds are good that his reserve back gas would not have been adequate to allow the much longer deco on back gas alone if the dive were deep enough and long enough to truly warrant two deco gasses.
In this case, the diver would have to rely on the deco gas reserves of other team members and may now be doing this in a situation where the team is now outside the original plan due to the effort to switch regs (in the case of the switch occurring with a low percentage deco gas at depth).
That's a very poor position to put the team in. Consequently, I might consider a reg swap but only if it can be done without risking both deco gasses, and only if it can be done after transitioning to a lost gas scenario and does not further delay the dive plan and team. In the event of a reg failure on the first deco gas, that criteria will not be met, unless it's a five bottle dive with an empty bottom mix stage that can donate a reg.
It's an advanced sidemount cave diving class and that implies several things about the entry skill level. It still took the student 5 attempts over 2 days to successfully swap regs from one sidemount tank to the other.
It's also not stated but it is a last ditch "the gas planning wet seriously wrong due to serious dealys in the exit, and I've also had a reg failure" scenario. One of the positives of sidemount diving is that you are in essence diving independent doubles and, properly managed, there is enough gas in either tank to get you out of the cave or at a minimum back to your first deco gas, stage, etc so it virtually eliminates the possibility of total gas loss.
The qualifier here is that sidemount entries usually result in low viz or no viz exits that are much slower, so "thirds" gas management is not enough in terms of conservatism. In other words, a low viz exit is not an "emergency" but rather the norm, and one that should not require you to use your reserve gas. Consequently, if a reg failure poses a problem, the problem is not the reg failure but rather a lack of sufficiently conservative gas planning.
Planning to swap your sole remaining sidemount reg from one tank to the other in anything otther than a dire last ditch situation, would again just be adding one error in judgment/planning on top of others.
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In either case you need to think long and hard about the logic of switching a reg and you need to consider the systemic impact of the attempt on the dive and on the team, as well as on other options that while they might lengthen the deco, are much more of a sure thing and represent a far better contingency plan.
