In Michigan trailer behind a trailer is not permitted, doubles require a fifth wheel on the lead unit. Can't speak for other states. I downloaded the reg's from Florida DMV site back when I was researching where I could tow them, (That's why we now have a Voltage). According to the DMV of Florida, doubles are permitted if the unit is commercially licensed but only on roads that provide a 12 foot wide lane or within 2 miles of said road. Was so confused by what I read that I said screw it and towed doubles in a few years ago. No trouble going in, (guess they wanted my tourist dollars), but got stopped on the way out. After a lot of finger wagg'en officer let us continue but said we were in their "puter" system and if we got stopped again they would probably make us break them up. Guess he thought my class "A" operators license with triple trailer and Hasmat endorsements was good for something.