This isn't my area of expertise (not that I even have one) but why? Are you just looking to teach students about how to work with doubles in a pool or something along those lines?
My first thought is that my old double 72's don't weigh much more than a large steel single. The length and trim are similar to big doubles. They are just way lighter than even double AL80's and have great buoyancy charistics, much better than AL tanks.
I'm not condemning, just truing to understand the objective.
It's OK to question/condemn me. I don't mind!
Here's my deal: most of my customers are tiny Japanese people with ridiculously small lung volumes. This means they might even struggle with the weight of an 80 even as an experienced diver. Not because they are weak but simply because the balance is completely wrong. Hanging a tank that weighs half their body weight off their back is hard, and they generally have no "front ballast' to make it any easier. (It's probably best to think of some skinny twelve year old boy you know and imagine him in twin tanks of normal diameter. It's a little bit of a question of strength, but mostly a question of balance. Normal size tanks hang too far from their bodies. You can get them to put up with it, but if you can dive them in pony bottles it just works out better. They do not need the air, and putting the weight nearer the body, because ponies are smaller diameter, matters because they have no front ballast, whether is it a belly or even just upper body muscles mass.
They never need to dive 80's anyway because they will come back with much more than half a tank on any dive they do with diving an 80 with me. I can do intro dives with Japanese people using 30 cf pony bottles. We definitely can use a 50 or 67 with two or three different customers.
Some of these customers see the tech setup and and get curious. If I put them in twin normal size tanks, they would never be interested in going any further. The balance of a larger tank combined with the weight would make them think of anything but fun and adventure.
On top of all that: One of the very real difficulties in teaching Japanese is that they never use enough air to actually ever need to look at their gauge during training or diving, ever. Any single tank of normal size never ends up more than half empty on any dive. Even ridiculously small tanks like 50s can come back half full on deep dives in heavy current. (One of the things you will see on any Japanese dive boat is the divemaster tank checking rather than swapping out tanks. Americans seeing this get confused because 'you always change tanks between dives'. But the Japanese don't need to, so often 2 tank dives leave the dock with less than 2 tanks each.) In tech diving, not being used to
really needing to look at your gauges is a REALLY BAD THING. If I had twin tanks they could actually use up, then 'training' them to use and rely on their gauges when they actually had to look at their gauges monitor air supply would be nice.
Stories about unbelievable small lung volume: I have had a student diver take more than a full minute to drain the residual pressure from the hoses in a reg with the valve shut off, in that confined practice drill where we shut off the air and have them get used to the feel of breathing from a near empty tank. I can drain the same hoses with one deep breath in two seconds. Don't even get me started on trying to teach small lunged people to blast clear a snorkel. Many of them simply do not have the lung volume to do it, and they have to really, really work at snorkel clearing.
This whole thing is for their experience of diving with doubles with a 'real' tech setup. All my customers are tourists: them taking home a picture of them diving twin tanks with a necklace reg is planting a seed for the long term. Most of these people would will never dive again, even recreationally, ever. They just get certified on vacation because the company vacation has everyone doing it. It is my job to pack as much fun and adventure into their vacation so that they change their vision of the future to include diving. Turning intro divers into open water students is something I can do, because I can present a challenge to them. But making sure these newly certified divers future challenges is the next step.
Right now the Japanese tourist market is exclusively recreational, and dominantly intro. Someday it would be nice to have planted enough seeds to have some of these divers want to do some tech diving or more likely some GUE-type training, since that course seems designed with the tourist diver in mind.
If you're hell bent on doing this use at least 30's, that's plenty ridiculous enough.
