As a general rule, this forum really hates towing tandem. You will get lots of comments (some already) about how you shouldn't do it, not safe, insurance won't cover in an accident, the list goes on.
If I were you, I would decide which states are most important for you to tow through, and directly research the rules for those states. I would not rely on most of the information that will be posted here, or any on-line listing that claims to have info for every state.
I've towed tandem for over 20 years now, and have well over 100,000 miles under my belt doing it. What I have found is that it is legal in most states - except those on the coasts.
I have taken my rig through 7 mountain/mid-west states, and I am over length in every state (including my home state) with the exception of MT. It is my experience that if your rig handles well, and you are driving safely, LEO does not care about length.
That said, you should still know the laws, and be willing to suffer the consequences if you go outside them and have a LEO throw the book at you.