Regarding the primary donate: I understand that this technique was developed and found to be optimal for a very specific user case: diving in caves, in narrow passages, where a very long hose is the only way of supplying air to the buddy, and in a team approach where all the divers are trained and equipped in the same way (and are high-level tech divers). I agree that this is the best way in these circumstances. . . . But consider a more common case: recreational unexperienced divers, employing a primary with not-so-long hose, which of course is routed "correctly" for the owner, but comes out to be "wrong" (upside down) when given to the buddy. In such case, I think it is better to have a dedicated "buddy reg", on your wrong shoulder, with a longer hose than your primary, and ready to become correctly oriented when donated. This is NOT your alternate air source. On my cylinder I use a standard reg (primary plus AAS), both routed correctly for me. And, on the second post, I mount another separate reg, which is the one designated to be donated (and NOT to be used by me as an alternate air source).
It may ultimately be best to split off the 'primary donate' discussion to another thread. But, since it has been raised in the context of THIS accident, as a possible contributing factor, I will respond in this thread.
First: I do not know the specific hose lengths being used by the divers in this accident. I do not know if the divers considered themselves to be using 'primary donate', or if donation of the husband's primary was the result of the OOA diver reaching for it. I do not know if they practiced alternate air source use routinely, or if ever. As I said earlier, I don't think the hose lengths, or 'style' of donation were the 'issue' in the outcome. I think failure to monitor gas supply (by BOTH divers, according to Anthony's summary of the video evidence in post #147), and panic, and procedural / physical errors in the exchange were the issue. That is the primary lesson we should learn in my opinion.
Second: any procedure, and configuration, must be familiar to both divers involved, and panic can render
any procedure and configuration useless.
Since the accident did involve an attempt to share air, it is not unreasonable to discuss the issue of how best to share air. 'Primary donate' is both a procedure and a supporting gear configuration. The procedure is something that should be regularly practiced, so that it is fluid, facile and timely in execution. The gear configuration involves optimal hose lengths and optimal second stage performance,
I will fully agree, that donation of a second stage on a short hose, whether it is the primary or alternate, is problematic. So, when a diver adopts the practice of 'primary donate', s/he must also equip themselves to do that efficiently, effectively and safely.
In what has come to be viewed as the 'standard' regulator configuration, the primary second stage is usually attached to a 32 - 36" hose. The alternate second stage is usually attached to a 40" hose. That extra length allows for some flexibility when sharing air, as the 'donated' second stage is attached to a hose long enough to allow for comfortable positioning in the mouth of the OOA diver. Notably, the hose is still routed over the donor diver's right shoulder, and and it must be turned 180 degrees to 'face' the OOA diver, when the two divers are oriented 'face to face'. If the primary second stage is donated, the 32" hose may require the divers hold each other even closer, so that the second stage will stay in the OOA diver's mouth without issue. If the two divers are not panicked, and execute the donation of a second stage in a deliberate and conscious manner, even a 32" hose length can work effectively.
I can take a 'standard' regulator set-up, and configure it for primary donate, by using the second stage, on the longer hose, as my primary regulator, and putting the alternate second stage, on the shorter hose, in a position where I can quickly and effectively access it after I donate the second stage in my mouth to an OOA diver. I prefer that position to be on a bungee necklace, under my chin, where I can almost put it in my mouth simply by turning my head down. I always know exactly where it is during a dive. I do NOT have to change hose lengths to make this work. And, I teach divers this practice. I also teach them that they should have two equivalent (in terms of performance) second stages rather than a 'good' primary and a cheap, needlessly de-tuned, lower performance alternate. The extra cost of a good alternate is, frankly, trivial in the grand scheme of things. (And, I consider the excuse that I sometimes hear from divers - 'I can't afford the extra cost' is nonsense.)
Primary donate' is sometimes, unfortunately, also equated with a 'long hose' configuration. It does not have to be. Yes, a long hose configuration with primary donation evolved from cave diving, and allows for in-line swimming through restrictions. But, it can also be used for, and has value in, recreational diving. I have used a long primary hose (7') for years for all of my recreational diving, as has my wife. It is an easy configuration. It is very easy to donate, it is very easy to orient the donated second stage without any 'torqueing' of the hose. And, she and I practice it on EVERY (single, solitary) dive we do together, for at least a few minutes - that is how easy it is to use. It allows us to easily swim side-by-side during dives, when we are sharing air.
Adding a third LP hose, routed over your left shoulder, and a third second stage, is fine, if that works for you. I don't see the need, and I don't see it as being particularly streamlined, but I certainly won't say it is wrong. It is certainly not a commonplace configuration, but as long as you brief your buddy, that shouldn't be a problem. Yes, if it comes over your left shoulder, AND the hose still is attached to the right side of the second stage, it won't have to be turned 180 degrees, and that is a benefit. I personally want to keep my left shoulder space as clean as possible, so my inflator hose is the only item coming over the shoulder. But, if you choose to put that third hose there, and you can easily and quickly deploy the second stage, and manage your inflator hose without hindrance, that is great.