Wanna do it in doubles? Well, "go dive doubles for a while and figure it out on your own so you're comfortable and competent in them before showing up to class" seems to be the "answer". Now, having done the book portion of tec-40, I'm aware that diving doubles isn't just "having another tank" and "valve drills" etc are a thing... so why isn't there a class to teach this stuff to people like there is for sidemount?
I do not really see the difficulty of using a back mounted twin set.
When I started diving in 1975 the standard tank here was a 10+10 liters double, with double valves and reserve.
There was no octopus, you had two independent regs on two posts. And no SPG, but the reserve acted partially as a separator manifold, as it did close one of the tanks at pressure below 100 bars.
I did always find that a twin set of proper size (9+9l alu, 10+10l steel) is more streamlined and confortable than a fat 15 liters tank.
Learning to use two independent valves is trivial.
And nowadays the separation is also much easier to manage than using the reserve valve, as you simply have a central third valve which can be closed in the rare case you need to separate...
So I really do not see the problem. Rent a reasonably-sized twin set and dive normally...
At our times the max depth for recreational diving was 50m, now it is just 40m. And light deco was also standard for rec diving, providing that the dive plan did not require any gas additional to the back tank.
So we did use a 10+10l twin set. But now, with 40m and NDL limits, just rent a 7+7 liters, 232 bars tank: you will get slightly more air than with the standard 15l, 200bar single, in a much more streamlined and lighter package. With it, you will learn how easy is to dive doubles...