what are the laws concerning towing "doubles" there....
some states allow commercial trucks to tow doubles, but not passenger vehicles....
some states allow towing a second trailer ONLY if it is connected behind a 5th wheel type trailer, and not behind a bumper pull trailer....
there can be different total overall length restrictions from one state to another.
as to there being a ready to install "kit", i dont think there is.... but its so easy to install a hitch on the rear of a trailer that there really is no need for a kit.
what one needs to do first is to make sure the trailer frame is strong enough, which it usually is, but dont depend on what may be "usual", and make sure YOUR trailer frame can handle it...
one only has to look under the rear of some of the entry level motorhomes and see how the chassis frame has been extended a couple times to create more length for the flooring of the unit, and notice how the welds are done by an amateur/beginner, one would know what type of shoddy work is acceptable to some manufactures... AND as its a motorhome, it STILL has a hitch at the rear of the unit for towing a full rated load.... one has to wonder at the marvels of "local" engineering at times...
if one were to weld some hitch tubing (5-6 feet long, each side) under the main frame rails of the trailer (with cross bracing) to spread the load over more area for reducing acute stress loading, and then build the hitch to these added rails, you would be good... but in my opinion, not with a very heavy trailer behind an rv trailer.... and if the lead trailer is a bumper pull, then it causes other issues when the last trailer is too heavy or loaded improperly, which can be true behind a 5th wheel trailer also, but is amplified in a bumper pull trailer.... which is why some states dont allow a second trailer behind a bumper pull trailer.....