Beyond not knowing whether every state will register such a trailer (assuming your state needs to), there appears to be a big question about the legality of the 2 point hitch system.
This was on Swivelwheel's website...
A frequent question asked is if the Swivelwheel system is legal to tow in all states. There are basically 22 states that do not allow double trailer towing, which some call "Triple Towing". These states are the entire east coast from Maine to Florida, Oregon and Washington. We consider the system an extension of the fifth wheel chassis frame. We do not consider it a trailer. The system cannot jackknife or fishtail. It is not like another ball mounted trailer swinging from side to side. Some states may not have that same view.
They are suggesting that it's not a trailer because of the fixed, double hitch system which has limited or no movement. Can't speak to other states but, somehow, I doubt that my state would agree with that.
If it has a wheel on the ground and connects to a hitch receiver, they're going to call it a trailer in this state.
Sad, because I actually like the idea. Just have my doubts that I could do such a thing here.
Point is, I guess, to check the laws in your state before doing it.