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.
So, if you can activate the brakes on that pickup, then I reckon the trailer brakes would activate from the hitch connection. So, if they have a toad break system in the pickup, it could be fine for the normal braking you would see.
Of course if something hinky happens up front, who knows how all that is going come to a stop.
I agree. I don't wanna be in front, or beside that rig when he has to lock 'em up.
IT might be interesting to see from a distance.