Sheesh, that's a good question. Here's the context
Michigan Vehicle Code, MCL 257.721 subsection (3) wrote:
(3) A vehicle or trailer towed or drawn by a vehicle shall be attached to the vehicle with forms of coupling devices in a manner so that when the combination is operated in a linear alignment on a level, smooth, paved surface, the movement of the towed or drawn vehicle or trailer does not deviate more than 3 inches to either side of the path of the towing vehicle that tows or draws it. The vehicle or trailer shall also be connected to the towing vehicle by suitable safety chains or devices, 1 on each side of the coupling and at the extreme outer edge of the vehicle or trailer. Each chain or device and connection used shall be of sufficient strength to haul the vehicle or trailer when loaded. In the case of an implement of husbandry with a gross vehicle weight rating or gross combination weight rating of 10,000 pounds or less, the safety chains or devices required under this subsection shall conform to the federal motor carrier safety regulations requirements contained in 49 CFR 393.70(d)(5).
I've never seen anyone with safety chains setup that way, they're on either side of the coupler on both the trailer and vehicle. Must be something that's seldom enforced.
Take note, it doesn't stipulate to double-towing, it appears to apply to any form of towing with a vehicle and a trailer?
Subsection (5) talks about double towing.
subsection (5) wrote:
(5) Notwithstanding subsection (1), a pickup truck with a towing rating equal to, or greater than, the weight being towed, equipped with a fifth wheel assembly that conforms with the standards prescribed in the motor carrier safety act of 1963, 1963 PA 181, MCL 480.11 to 480.25, towing attached with a semitrailer designed for recreational living purposes may tow an additional trailer or semitrailer under the following conditions:
(a) The additional trailer or semitrailer shall be attached as provided in subsection (3). The safety chains described in subsection (3) shall be securely attached at the extreme outer edge of the attached trailer or semitrailer with a locking mechanism. The towing vehicle hitch shall be of substantial material and shall be attached in a proper and skillful manner to the frame of the towing vehicle.
(b) The total length of the pickup truck, semitrailer designed for recreational living purposes, and additional trailer or semitrailer, and load, shall not exceed 75 feet on any highways in this state.
(c) The gross weight of the additional trailer or semitrailer towed or drawn shall not exceed the empty weight of the pickup truck or the empty weight of the semitrailer.
That would seem to indicate the folks living the high life driving their huge DP that is towing a truck that is towing a boat are driving an illegal combination, as the DP doesn't have a fifth wheel assembly.