If your predetermined intent is to run away and save yourself when there is an emergency, then I'll say again:
You have no business diving with a buddy in that environment.
Doc and all,
I want to address this issue from the point of courage and cowardice because I think something is being overlooked. When I posted this question, in my mind, I was not thinking that the diver in the donor position was a coward. In fact, in my mind's eye, the imaginary team was one in which the donor was actually the type of individual who could make the hard choices and not the kind of man who would cut and run.
The ability to make hard choices is often what leadership is about. For those who served in the military, think of the finest officer and NCO you ever knew, or if you didn't have great leadership, think of the type of person you would be proud to serve under. I'd bet that individual would have the courage to sacrifice himself for others as well as have the courage to sacrifice others for the greater good. Could such a leader look someone in the eye and say, "I need to let you die, because ... if I don't close this hatch, this compartment will flood too." But, I bet that leader would gladly swap places with the sailor or sailors on the other side.
If a lifeguard did everything possible and gave his all to make a save, is he cutting and running when he realizes the only way he will live is to let the victim go?
In an air share, you reach a point on the exit where you know you normally use 500 psi to exit on your best day and there is 200 left in the tanks and a difficult restriction ahead, or whatever would make you certain both of you aren't going to make it. You are 100% certain, and not only are you 100% certain, you are 100% correct. You may not even make it and it would take all of your skill, all of your tricks, all of your experience, and to make it out would be a miracle in itself at this point, but you are good enough, that maybe you just might, but in that air share there is absolutely no way. It's not Ginnie. It's not a popular cave. It's in the Bahamas. You know the only three cave divers on that island and they are not planning to go diving that day. Are you really a coward? Are you cutting and running?
It's a hard choice. Hard choices take courage.
I read a psych study that was done in the 1960's on a group of BUDS/UDT students in training and a group of US Army Special Forces soldiers at an A Camp in the highlands of South Vietnam. The study was measuring stress hormones. IIRC, they found that the stress hormones of the BUDS students were normally higher because they were afraid more often than the green beret A-team in combat. As different pieces of equipment would be introduced, even benign things such as when a facemask was added, once trainees became used to the swims and drills with fins, the level of cortisol would measure higher than those experienced by green berets on days when they were under fire. However, the stress of the green berets would be highest just before a mission, but lower in combat. What they discovered during the interview process was that the green berets were more afraid of letting their buddies down or making mistakes that would cause their teammates to die than each individual was afraid of his own harm or death. Coincidentally, the BUDS/UDT students were afraid of failure and washing out of the program. The conclusion was that the fear of the unknown, the fear of letting others down, and the fear of failure physically released greater stress hormones than were released by those involved in battle. In fact, the lowest levels of stress hormones were on the days when the green berets' intelligence reports told them to expect attacks against the camp. (I know some military guy is going to want to make a military intelligence joke at this point so I'll preempt it!

Playing Devil's Advocate here, so LeadTurn doesn't have to become my thread secretary, could it be that the "easy way out" is just to "quit" by sticking with a buddy knowing that you don't have a prayer, but it is easier because you don't have to make the hard choice and you won't have to face ridicule?
Would it take more courage, knowing 100% for sure that two cannot make it, to decide to try to save one?
As MafiaJoe said, correctly, "He's already dead."