I'm mainly a vacation diver who dives the Caribbean when I get a chance. 6'1" tall, close to 275 lbs, bit of an air hog, ... Don't see my SAC getting a whole lot better anytime soon. ... I use a Sherwood Avid 2x BCD, Atomic B2 reg., and I don't have any experience with back plate wing setups.
The background helps us understand your question.
could you stick a spare reg. on a 2'nd tank, then connect it off your side with a strap or something (I'm thinking more like a pony bottle maybe than a typical side-mount setup) while having your tank on your regular jacket-style BCD, single-style? Maybe call it 'red-neck side-mount?'
Yes, you can do this. A number of people do it, but refer to it as diving with a 'slung' pony bottle. If I recall correctly, the Avid has chest D-rings on both sides, and single D rings on the bottom of each side of the waist strap, so there are attachment points available. There is a lot to be said for a thorough discussion of 1) gas management practices, and 2) training approaches to improve your SAC, and 3) how to further develop your skills. And, you have received some good feedback. But, your specific question related to whether or not you could do this, and whether other people to it, and the simple answer to both questions is 'yes'.
Is the 'red-neck side-mount' option something that some people do and works, or dead in the water?
As noted above, some people do it, but may call it something else (diving with a 'slung' pony bottle, for example). There are a number of threads on SB that debate using a pony bottle as part of your primary gas supply, and I encourage you to check those as well.
Been to Bonaire 4 times; hope to go back in the future. Getting hold of big tanks isn't practical, but 'all you can eat buffet' style 80 cf aluminum nitrox tanks, no problem. I'd like to take 2 tanks in with a separate reg. on each, breath each down to around 400 - 500 PSI (shore diving), then come out, basically.
Several posters have expressed concern about the possible consequences of the approach of breathing the tanks down to only 400 psi, and I understand where they are coming from. But, Bonaire diving easily allows this. Most (not all, but most) of the dives are very close to the shore and this option is what a lot of people do - they dive (a single tank) til they hit 500 (or 400, or 300, or ...) then surface and swim in to shore, or board the boat if they are doing a Kleine Bonaire dive or one of the less accessible northern dives, for example. Whether this should be encouraged as a general practice in other environments can be debated in another post, but in that particular environment (Bonaire) it works. You said it yourself - your practice is to go deeper at the beginning then ascend to a relatively shallow depth toward the end of the dive and finish off the tank.
My thoughts are fairly close to Herman's comments. You can do it, but it is quite possible that you will add drag, or alter your trim in a way that increases your air consumption, to the point that it is a self-defeating proposition. That doesn't have to be the case, but it may turn out that way. Three comments: 1) like several previous posters, I don't notice any effect on my trim when I dive a single back-mount tank and a 'slung' AL40. Changing from my backgas reg to my pony reg isn't an issue, and I learned to do that smply by doing it - no formal training at the time (that came much later). I DID, however, notice the effect of a 'slung' AL80 on my trim when I first starting carrying one, even with back-mount doubles. I perceived a slight tendency to roll to the left a bit, and spent more energy 'station keeping' at first than I thought I would. Now, I don't notice any effect, even when I sling both an AL80 and an AL40 on the same side. But, I mention it to alert you to the possibility that you might notice that at first, IF you use a back-mounted AL80 and clip a second 80 cf tank on the side; 2) I have taken my sidemount rig to Bonaire and used it with two AL80s for some lo-o-ong, shallow (40-60 feet) reef dives. The gas supply frequently exceeds my interest in staying down, but that is just me. I get a lot of enjoyment from single AL80 dives down there; 3) a real concern with what you are proposing to do is whether you will inadvertently exceed NDLs, and whether you will know what to do if that occurs. Again, a subject for another thread, perhaps, but something to keep in mind.
My suggestion - it certainly won't hurt to try it. I would definitely try 'red neck sidemount' out in a controlled environment first, e.g. go to a pool, and work out the logistics - how to attach the bottle to your BCD, how to switch regulators, how to stow you slung bottle regulator when not in use, etc. - before heading off to the Caribbean and trying it for the first time. And, since 80s are a lot more available than AL40s, work out how to do it with 80s, even if you first try it with a 40. One of the great things about sidemount diving is that it is associated with a lot of do-it-yourself behaviors. People start diving sidemount for a variety of reasons, they may not go with a commercial SM rig to do so, and they simply improvise, to find what works for them. That part is fun. True sidemount configuration is a stable platform, is very relaxing to dive, and is quite appropriate for recreational diving, not just more technical diving involving deeper dives, in overhead environments, with decompression obligations. What you propose to do is not irrational or unrealistic. Nor is it something that you should approach in a cavalier manner. Preparation, experimentation - probably involving some trial and error - in a controlled environment, and a thoughtful approach will work wonders.
