@Jfinch An excellent question. Keep thinking and asking!
The incidence of death from equipment failure in scuba are extremely low, especially in the recreational world. I personally have never seen even an incident that prevented gas from being delivered to a diver. Therefore, again recreationally, where the surface is always available to you, it’s basically considered too much equipment to offset such a small risk. Your buddy has your extra gas, and if all else fails, make your way to the big tank in the sky.
As your dives get more challenging and the environment becomes less forgiving, additional redundancy becomes more reasonable. As has been mentioned, many people carry what is called a “pony bottle“: a small, completely separate tank and regulator set up that can be used in case of emergency.
As dives become even more challenging, it may become impossible to get to the surface for a very long time, such as in a serious decompression or cave dive. In that case, it is standard to use multiple tanks. Some people use completely separate tanks, such as in sidemount (a form of independent doubles), or tanks that have been manifolded together with an isolation valve: backmount doubles.
Most people would say that such equipment would only be used in such heavy duty technical situations. Personally, I dive doubles on every single dive I do. Of course, I also have some fairly small doubles that I use for straightforward dives.
Like I said, I think that was a great question that you asked. Hopefully you have gotten some answers that will help you to understand what is available. In addition, I have a couple of thoughts along that line for you to ponder if you wish.
While it may seem really important to have the extra redundancy of extra tanks and extra regulators, consider what *actually* kills most recreational divers. It’s not equipment failure. It’s simply running out of gas. And fixing that requires no extra equipment. It just requires paying attention to your gas supply and having a proper understanding of gas management. All the extra regulators in the world won’t help if you don’t pay attention to your gauge and leave proper margins. You can Google “rock bottom gas management” for more thinking on that.
And a related thought. Most new SCUBA divers — and plenty of experienced SCUBA divers – tend to focus on equipment. That includes trying to solve problems with equipment. In most cases, though, the problems that need to be solved are created from lack of ability or training, not lack of equipment. There’s even a saying for that: “equipment solution to a skills problem.” Something to consider: if you can foresee a problem with a particular configuration, it’s likely other, more experienced divers could as well. So ask yourself: why have they not considered your solution to the problem? Or more specifically, how are they solving the problem without adding additional equipment? Personally, I find when I ask that question, it often opens my thinking up to completely different avenues.
None of this is to take away from you or your question. Seriously, I like the question. Keep thinking about the Why. Too many divers focus on the what and ignore the Why. But, there can be multiple levels to the Why. So, keep thinking, and keep asking!
