From quick lookup, it's not allowed in NJ outside of a commercial license with a tandem saddle and commercial trailers.
Here in WI it's allowable for private recreational use only, the first trailer must be a 5th wheel and you still cannot exceed 65' total length.
Michigan has an interesting add-on to a standard drivers license, recreational double. Answer a couple questions and you're allowed to tow two trailers, as long as you keep it within the limits and it's recreational purposes only.
Speaking from experience, I hate double towing. Plan the ever-loving crap out of your driving route, maneuverability sucks and reverse is impossible with two trailers.
You'll also want some way of easily maneuvering the rear trailer on to it's attachment point, such as a trailer dolly or one of those trailer jacks with a drive wheel.
Another tidbit pertaining to the recreational side of double towing. Not all campgrounds have drive-thru sites, you may have to separate the train on arrival.
here's two semi-useful links
http://towingworld.com/towinglaws.cfm
http://news.pickuptrucks.com/2009/0...you-in-many-directions-if-you-double-tow.html