DOUBLE towing (truck/trailer/trailer) is legal in Montana and several other Western states. Yes, I could tow the Jeep and the boat behind the motorhome, legally, except that she-who-must-be-obeyed will not allow it.
TRIPLE towing (truck/trailer/trailer/trailer) is legal in Montana on the freeways and within a certain distance of the freeway to allow getting to the yards. It is usually only seen with Commercial trucks (UPS, FEDEX, etc.)
Maximum length is, IIRC, 75 feet whether commercial or non-commercial.