In some states it is legal to pull a 5th wheel and have a trailer behind that. (There's something about how it relates to a semi pulling a tandem trailer). But, those same states won't allow two trailers hooked together when pulling from a standard bumper (or below bumper) hitch.
I also found it is legal in some states to pull a trailer and have a small "tool on wheels" type device pulled behind that. Think towable generators, wood splitters, chippers, etc. There are several tree service companies that make use of this to haul both a
chipper and a trailer that collects the chips.
Towing laws for all 50 states here:
Towing World Official Website
States that allow double towing:
State Laws Can Pull You in Many Directions if You Double Tow - PickupTrucks.com News
Ugh. I just realized that when I tow my 26' snowmobile trailer with my 26' motorhome, it isn't legal in Oregon (where I live) unless I have a special permit. Funny how the length matters so much more than the combined weight (at least in OR). It's the stopping that's the hard part (as I discovered when the battery went dead on my trailer brakes and I didn't realize it).