I was thinking more about something like an Air-2/SS-1. If the donor doesn't first switch to his alternate, things would get exciting quickly if he's just mugged for his second stage and isn't good at reg switches. The OOA diver taking the alternate isn't really practical, since there's usually less than a foot of available hose on it and it won't rotate forward enough to use.
OTOH, I never had a lot of confidence that a real OOA diver would actually go through the process of signaling they're OOA and asking to share and giving the donor time to do a reg switch/clear. Just taking the reg seems much more efficient, although it does mean that the donor (possibly unaware of what's going on) needs to be ready to switch to his alternate at any moment.
I can't decide if it's good or bad or just a reflection of reality, but thanks for the explanation.
That is the reason that BSAC
requires that its instructors teach having the OOA diver take the alternate from the donor rather than having the donor provide it. PADI allows its instructors to choose which way they teach it.
Every PADI instructor I know tells students that each situation is possible, and this is something they should discuss with a buddy ahead of time. They should practice it with a regular buddy.
I show all three alternate air system my students may encounter while we are in the classroom and explain how each works. In the pool, we talk about what
might happen during a real OOA situation. What I tell them is that it doesn't matter what you have practiced in the past, the person who determines what will happen is the OOA diver. When working in a traditional golden triangle setup, I tell them that they have three possibilities:
1. The OOA diver will reach for the alternate, with or without signalling. In that case, get out of the way so that you aren't fighting over it. Then complete the proper process.
2. The OOA diver will signal you and then look at you, waiting for some help. In that case, donate the alternate and complete the process.
3. The OOA diver will grab for your primary regulator, usually without signalling. In that case, spit it out, take your alternate, and then sort things out once the panic is over. I have (incredibly) heard of instructors teaching students to shield their primaries in such a situation to prevent the OOA diver from taking it. I don't think an OOA situation with a panicked diver is a good time to start a fight.