Okay, one data point: My brother-in-law was stopped on his way to Minnesota with his fifth wheel and boat. He had made the trip several times before with no issues, but got stopped this time. He was told his overall length was 6" too long. He wasn't forced to drop his boat, but was told he couldn't continue, and would have to make other arrangements.
I towed tandem several years with no stops of any kind, even in states that don't allow double towing. But I was also aware that it could happen. It's simply a roll of the dice.
