I deal with commercial laws every day so just to be sure my answer was correct I ask my safety man about your question.
This is what he said..........
Commercial doubles are allowed in all 48 states and follow the rules I stated below.
RV's and other private vehicles have separate rules because they are exempt from things like CDL's and there is no way to enforce if the driver is qualified to drive such rigs. So the federal rules do not apply.
He give me the link below with the non-commercial rules.
You can see that non-commercial doubles are not allowed in Florida.
From the Florida Department of Highway Safety and Motor Vehicles web site.......
"Florida statutes require that non commercial vehicle combinations consist of no more than two units and such combinations may not exceed a total length of 65 feet. F.S. 316.515(3)"