Here are the towing laws for each state. Double towing is referred to as Triple Tow in their definition (not mine).
I double tow all over ID and MT with a TT. First thing to consider is how big is your boat? Mine is 16' with a 200# tongue weight and 2400# pull weight.
Next, don't even consider using that 2" factory receiver!!! Take your TT to a welding shop that does this sort of thing. Your existing TT frame SHOULD be reinforced before adding a hitch. Mine, for instance, was done by a local shop and rated at 300/3000. And he will not do it to just any TT. There has to be sufficient frame to start with.
I also have a rear view camera on the TT to "observe" and aid in hooking up.