are two situations that would require the use of an alternate air source.
Diving accident statistics show that a large proportion of cases in which one diver seeks air from another diver results in two deaths rather than one.
If you happen to be in cold water, two divers breathing off of the same first stage will increase the flow of air through the first stage, lowering the temperature of the regulator and possibly causing the first stage to freeze.
Since a second stage regulator that is tuned properly has a great tendency to free flow, most octopus alternate air sources or BCD alternate air sources have been de-tuned to prevent free flow, making them difficult to breathe. Even if your dive buddy agrees to let you share his air, he might very well decide that he wants to keep his primary second stage and let you breathe from his alternate. That will cause some precious loss of time while you fumble for access of the air source and leave you with a source of air that is difficult to breathe from. Especially if you are deep, it may be critical that the primary second stage is a high performance model allowing easy breathing but the alternate air source is deliberately made to pass air reluctantly to avoid free flow.
Lets look at the second situation. Your dive buddy runs out of air. The situation is reversed and you have to decide if you want to let him attach himself to you. It is a shocking surprise to many divers when they find that the person they are diving with, someone they thought they could count on in an emergency, suddenly becomes a dangerous adversary when they run out of air.
If he has a pony bottle of his own, he will pose no threat to you. If he doesnt and he comes to you for help, the chances are that he might be in a frame of mind that will be at least on the edge of panic. Thats not quite the same situation as when the two of you were practicing the moves on the bottom of the training tank. And even worse, if it is not someone you have trained with, you dont know what he is likely to do in his fight to survive and how he might put you in danger. If he comes to you for air, you can hand him your pony bottle and the two of you can ascend safely with no physical connection tying you dangerously together.
Ladies, I have been using the word he to refer to the divers in this discussion only because it is easier to write he than he or she each time. I have not forgotten that a large proportion of divers are ladies. Women divers have an increased amount of danger when they are diving because statistically they are usually much smaller and have much less bodily strength than their diver partners. That makes them vulnerable to being injured or killed by a dive partner who is struggling to survive and is only thinking about getting air into himself.
Some quotes on the subject:
Air Sharing and Out-of-Air Emergencies by Lynn Laymon
"Sharing air is a safe and feasible alternative when both the donor and recipient are trained, proficient and practiced and the donor has plenty of air to share. However, this is seldom the case. Many experienced divers would rather risk their lives making a solo emergency ascent than share air with someone they don't know or have little confidence in. And don't expect to find every diver willing to share air with you, especially if he is not your buddy and doesn't know you. Diving accident statistics reveal a sobering fact about cases in which one diver runs out of air and seeks help from another diver. If one diver dies, both divers often die. Sharing air is serious business."
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Rodale's Scuba Diving, September 2000
Gearing Up for Going Down
"If you run out of air at depth you'll be glad your buddy's octopus is a high-performance regulator. It is, isn't it? In reality, your buddy is probably low on air too, and his octopus won't do you much good. That's why many deep divers use a completely redundant air source - a pony bottle and regulator or Spare Air."
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Rodale's Scuba Diving, November/December 1998
Solo Diving Facts and Fears by John Francis
"The buddy system can foster a false sense of security. Being a buddy does entail the responsibility to attempt rescue, perhaps at danger to yourself. The co-dependent diver syndrome is, in fact, one of the strongest objections to the buddy system. No one intends it, but the buddy system can foster the dangerous idea that somebody else knows better and will take care of you."
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Dive Training, October 2000
Waiting to Inhale by Robert Rossier
"Regardless of whom you're diving with, real friends may be hard to find when you're out of air at 100 feet."
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Scuba Times, May/June 1994
Running on Empty by Bret Gilliam
"Sadly the record of double fatalities for divers engaged in buddy breathing is disproportionately high."
Last but certainly not least: Over 90,000 pieces of life support equipment have been recalled in the last 25 years. Of those, 48,000 have been since 1995, and in the last 2 years alone, there have been 40,000 pieces of equipment recalled.
