I believe that the National Laws govern tandem trailers on the interstate system...and it's allowed as long as you're under 65'. When you get off the interstates is when the state laws come into play. Note, however, I believe all require that the first trailer be a 5th wheel trailer (with the goose neck hitch that connects over the tailgate into the bed of the truck...or a semi truck arrangement).

http://www.hitchemup.com/statetowinglaws.htm