We have never towed tandem ourselves, but last week when we were on the freeway going to Boise, we came past a scary accident. It appeared that the boat being towed behind the older travel trailer had gone off to the side of the freeway and dragged the trailer along. The boat was in the ditch. The trailer was on its side at the edge of the freeway. The TV was turned around facing the wrong way, but thankfully still on its wheels. Lots of police presence and traffic backed up for a couple of miles. That one convinced me that I don't want to take the chance of towing tandem, even though we have friends who always take their boat behind the 5er.
In Idaho it is legal to tow tandem, but if you go across the border into Oregon, you had better have a place to drop that boat until you return. Can't tow tandem there.