[wingnut60]

Several states allow 'double towing' or sometimes called 'triple-towing. If not otherwise disallowed, the first and most visual item is length--most states also allow a maximum length of 65', others are up to 75'--so if you can pull it and it is less than the maximum length allowed in a particular state, then it is legal. Some states say 'only permissible behind a fifth wheel'--there are lots of little legalities to watch out for, but length is the most often cited as illegal if stopped.
Truck/fiver+boat, car, flatbed trailer w/ATVs, etc all may be legal. I have even seen a truck pulling a car pulling a flatbed trailer, and you see a lot of multiple cars being pulled from an auction sale.
All mostly legal, but really subject to the whims of the LEO that thinks something ain't right.
And, welcome to the forum--hope you post often.
Joe