Seen some pretty dodgy combinations pass through this area headed for Mexico sometimes a school bus towing another buss towing a small car.
Never known of any reg in any state that considers this as an acceptable, practice. but none the less double tow-bars are seen frequently. I would lay bet that nothing but the primary has brakes. Back in the 1990's at the end of a job we had done there I found myself saddled with 3 trailers to bring back to Texas pulling 2 trailers on the ground is legal but the front trailer is supposed to be a goose neck or fifth wheel, In the class 8 classification Triples behind a Semi is legal in some states 1 will be connected via the fifth wheel and 2 will be connected by a single or tandem converted dolly using a pintle and lunette connection.
I on the other hand was in my pickup the trailer I always pulled was connected to my truck by a pintle. I loaded the biggest trailer on my trailer then cut a hole in the back of my trailer and bolted a ball on it to pull the smaller trailer of course I had brakes on everything that was on the ground and didn't weigh 18,000 lbs so my diesel pickup thought it was empty