You're good to go in Utah, and most of our neighboring states. I've towed doubles a lot with bumper pulls, but am still debating putting a hitch on the back of my 5er. My bighorn, and virtually all of the other similar 5ers I looked at when buying this rig aren't at all overbuilt in the frame department. I'm not sure It's up to even towing my little utility trailer and RZR. I should just be able to keep int under 65 feet total length if I decide to go that route.
As mentioned, backing up is difficult, especially if the trailers are of significantly different lengths. You have to watch your turns a little more carefully as well. The biggest problem I ran into however was parking everything in the campsite. Big sites tend to be at a premium if you're a weekend warrior. Sand Hollow has a lot of good options, and you'll probably be okay at Flaming Gorge. Lake Powell depends on where you're looking to camp. Even at the developed sites you can always leave the truck and boat trailer in the launch parking area if needed so it's all good there.
Again, the only concern I would have is making sure the combination is solid enough to not end up damaging something.