Double trailer towing, is legal in some states, illegal in others. In fact, in Wyoming, you can double tow with a travel trailer (I don't know of any other state that allows a bumper pull to tow an additional trailer). The requirements vary from state to state. In Michigan, first you'll need a special endorsement on your license. That includes a written (no driving) test. Once you have the endorsement, you can double tow. The max length of the entire rig is 65 FT. The receiver/hitch on the fifth wheel MUST be connected to the frame and the safety chains on the second trailer MUST be attached to the extreme outside edge of the fifth wheel.
Other states have different requirements, and it's my understanding that states have reciprosity, but I've also been told that some of the Northeastern states do not allow any double towing, not even by visitors. I'd suggest, if you're unsure, call the State Police/Highway Patrol in the states you're planning to tow through just to make sure you don't get stopped and forced to unhitch one trailer. I've read in Trailer Life of some people being stopped and made to leave one trailer on the side of the road, tow the other to the nearest "safe place", unhitch and return for the second trailer. That's not the way I'd want to spend my vacation