Of course I am not from Texas, and I know that local use can be quite different around the world.
When I started diving there were only twin tanks available here. A single tank was simply carrying too little air...
I did start using a single tank only 10 years later, when 15-liters steel at 200 bars were made available, around 1985, providing the same 3000 liters as a 10+10 liters twin at 150 bars.
And even on single tanks I did always employ two fully independent regs, both with the same top specs, on two separate valves. This is what I did learn as minimum safety requirement during my first course, and I do not understand how people can rely on just a single valve, single first stage, and a sub-par quality octopus.
Please note that I am a fully recreational diver, no way I could be tempted by technical diving. So I do not see any reason for a certification about using double tanks, as doubles were the standard equipment used during my first course for training, and which I did use for more than 10 years, before switching to single tank. I did never read anywhere that standard OW or AOW certifications pose limits on the number and/or size of the tanks employed...
Please correct me if I am wrong, there are so many certification bodies nowadays that I could not know the rules for all of them.
But both of my sons are AOW-certified with PADI, and I did not see anything impeding them to use the old ARALU twin tanks which both I and my wife still own...