Yes, there are states that allow a "double tow". I'm sure each have a licensing requirement for it. As for the towing ability, there are many rigs out there that can legally tow that much weight (like mine, #15,000). It is safe? Now that's debatable.
Stopping that much weight in a hurry would be a white knuckle ride. I see many Semi's pulling 3 trailers across the desert (triple tow) and I am sure they are much heavier. The skill and common sense of the driver would be a major factor in that scenario.
I would also want to pass him if possible and put that set up in the rear view mirror.