I just checked the WI DOT website.
As of 2011, no special permit is required for hauling two trailers for recreational use.
No vehicle in the combination can be longer than 45', taller than 13.5', and wider than 8.5' with the exception of a motor home as the tow vehicle
Total combined length cannot exceed 65'
A vague regulation states that it cannot be operated in poor driving conditions
If the combined weight of both towed vehicles exceeds 3000lb, at least one of them must have functional brakes.
That's about all the regulations there are. Come to think of it, I have seen pop-up campers towing a second trailer from time to time also.