If you haven't figured it out already, this is one of the hornets nests on RV.net. Lots of fear mongering, and even more mis-information.
Double towing is legal in almost every state in the middle of the country. Less so when you get to the coasts. Probably worth checking your insurance, but I have never heard of one that does not allow it.
As has been said, you are probably better off finding a independent hitch shop / welder / custom fabricator. If I remember correctly, it cost me about $300 to have the hitch and wiring done on my current trailer. Last year I had it re-enforced with the intention of towing a larger boat (which I have since ruled out).
I still tow my 16' Lund with a 60 HP Merc. It weighs closer to 2500 pounds all in. But it does not change the towing dynamics.
Here is a photo with my Lund.
And one with my bigger boat (since sold).