Sticking to the original post, you can indeed tow double trailers on ball hitches in Utah provided you don't exceed the max length limit. You can even tow them at 80mph and be legal provided you're on one of the sections of freeway where that is the posted limit. That said, 80mph with the little tires of those trailers, and of course doubles being somewhat more prone to sway might lead to a little excitement. Since you live in Utah, I'm sure you have seen a lot of doubles being towed, in fact, about a third of the folks we regularly camp with do it, and couple using 5ers, the rest bumper pulls.
I towed a very similar setup for many years without any issues. Tow vehicle was a 87 2 door S-10 blazer, 3 inch lift, hopped up 3.2 crate engine. Trailer was a coleman tent trailer, lifted on a custom suspension with a heavily beefed up frame I customized for off roading. I had a little 16 foot aluminum boat on a trailer that I'd carry the 9.9 hp motor and 5 gallon fuel tank in when traveling. It towed really well, and I could even back the combo up to launch the boat with the trailer still attached more often than not. Without the boat I could take that truck/trailer combo almost anywhere. I even did the Shaffer trail with it when I was camping on the white rim trail in Canyonlands.
If you're in northern Utah, there are several trailer manufacturing shops that can look over your trailer and custom build you a receiver setup if frame is solid enough make it feasible.
Good luck