We have transported our Bilenky steel tandem on a Thule “hanging” top tube rack, hitch mounted on our minivan for over ten years with the wheels on. No issues with bike, rack or van. I would only do this with a quality hitch rack.
Hope that helps
This does help - thanks! And that's exactly the type of "hanging" rack I mean - hitch-mounted, but with the bikes "hanging" from their top tube. What kind of Thule rack do you have? Have you been able to put multiple bikes on it along with the tandem?