I towed double with my Ford Powerstoke that I used to have when towing cargo trailers out of Indiana factory. The setup we used a 34' flatbed gooseneck and usually loaded two trailers on it, then towed a 12-14 foot cargo trailer behind the GN.
What you are talking about is LOT of weight being towed behind your fiver. IF your frame on your 5th wheel could handle it which I doubt very much, then you are asking for the "tail wagging the dog" because of the weight whipping the 5th wheel around.
On the setup we used, the GN trailer frame was very heavy and could handle the effects of the trailer hitched to it by the special hitch that we welded onto the back of the trailer. Also, we also only pulled the lighter trailers double. But we could pull them across the Indiana and Ohio turnpikes at the speed limits plus with no problems.
Another thing is to remember that you also have to stop all that weight. On our triple axle GN, we had brakes on all six wheels, but the small trailer in the back they usually didn't have electric brakes, although we had the wiring run back for any trailer that we towed double that did come equipped with brakes. We never had any problems stopping, even when we had to lock them up when a car pulled out in front of us going when we traveling at 45 mph.
With all that being said, "Did I like towing doubles?"
NO WAY. My friend still does it and has over 300k under his belt, He still doesn't like doing it.
The advice from both of us is "Don't try to tow the weight you are talking about, a fiver and a car hauling loaded with a pickup."
I cannot print what my friend said when I talked to him about what you asked.