Depends on the state. Some states don't allow any kind of doubles towing and some states allow up to 75 feet. Some states allow two ball and some only allow pin and ball. Check your state for regulationsAlso, I believe that towing bumper to bumper is illegal, it has to be gooseneck or fifth wheel to pull a double. Again, lots of people do it.
That is an incorrect statement for RV's.Federal regulations permit doubles if the truck is a truck. Which means that if a full trailer can be pulled by the truck (fifth wheel, or gooseneck) then a semi trialer is allowed if the semi trailer is legal with lights, brakes, safety chains and whatever.
Since no State my enact laws which restrick inter state commerce, you are good to go with a legal comination.