Depends on the state, in most western states it's legal. Head east. Not so much.
Example
Virginia, the maximum length for double towing, specifically a truck pulling two trailers, is 28 feet, six inches for each trailer on interstates and designated highways
Colorado, the maximum combined length for a vehicle towing two trailers (double towing) is 70 feet. This includes the length of the towing vehicle and both trailers.
I find that funny, I have a 22' truck plus 28.5 trailers puts me at 79' I can legally be 9' longer in VA then CO. And would be way less comfortable in a state with all those curves on the roads.
You can even have passengers in the fifth wheel in some states too.
Even more if you give them a means of taking to the driver. (walkie talkie)