The forum really is slow when you have this few of comments on what is always a hot topic that causes many to get their panties in a bunch.
I've towed a boat tandem behind my 5er since about 1999. I have not towed through the states that you mention, but I'll share my experiences and you can take them for what they are worth.
Probably goes without saying, but towing tandem isn't for everyone. You really need to think ahead (stopping for fuel can be challenging), backing up is very difficult, and it is very hard on your equipment. I had a hitch custom fabricated to the back of my 5er, and then had it re-enforced later. I am sure I have towed tandem for over 50,000 miles. During that time I had the shackles holding the axles of my first 5er fail, I had the first hitch on the back of my 2nd 5er crack, and the weld on my fold away tongue on my boat crack. Maybe some of that would have happened without towing tandem, maybe not. Moral of the story, keep a close eye on all your components.
Towing tandem also degrades the ride within your truck. When you add the boat tongue weight to the back of the trailer you lose some pin weight (which never really was a problem), but you also create a fulcrum, and every bump you hit is magnified. Truck hits a bump, it bounces, then the 5er hits and it bounces, which bounces the truck, then the boat hits which bounces the 5er which bounces the truck. A good bump will go through the train a couple of times. I can pretty comfortably tow the 5er solo 12 hours a day, but add the second trailer, and at the end of a 12 hour day I am really wiped out.
I have not towed through any states that prohibit the practice, but I have been over length in most states. What my personal experience is (at least regarding that) is that LEO do not enforce that regulation.
The boat that I towed the most was a 21' sterndrive that I kept for about 10 years. It was about 5500 pounds going down the road, and is the most that I personally will tow. To me, the limitation isn't what the truck is capable of towing, it is the limitation of the strength of the frame on the 5er. I sold that boat back in 2014, replaced with a 23' inboard that weighs 7000 pounds, which I towed tandem once, and won't do again. When we go camping at the lake with that boat now, we take two vehicles. I do have a 17' Ranger fishing boat that weighs somewhere around 3000 pounds that I still tow tandem (which really is a LOT easier and much less noticeable).
To me, at least with a 5000 pound or less boat, towing tandem is still worth the trade offs of either not taking the boat, or taking a 2nd car. But, like I said, it isn't for everyone.
And to those that will tell you that you are at risk in an accident, and your insurance will deny a claim, I simply say read your policy. Insurance is regulated, and if there is a peril that is not covered it needs to be listed as an exclusion. I have never seen a policy exclusion of either towing a trailer tandem, or something that would deny a claim for an accident while breaking a law. Quite simply, insurance covers accidents, and it doesn't matter if you are towing tandem in a state that doesn't allow it, or towing over the weight rating of your truck, or speeding, or drunk (or all the above), an accident will be covered up to your limits of liability.
