If you google for LeMond tandem you'll probably come across this image:
http://www.valleybicycle.com/images/lemond_tandem.jpg which would probably qualify as an "open frame" design
The history of my bike is a little bit more obscure. I responded to a message on the T@H list which said basically "my friend has a LeMond tandem that he wants to sell." I recognized that this was something unusual, and started the process of purchasing.
As I'm told, the seller / builder worked/works? for Trek and this bike was built to try out some various ideas. It's got the basic geometry of a mid-90's Trek T100/T200, but the main tubes are Reynolds 853 instead of True Temper. I think they were having a bit of fun, and applied LeMond labels on the frame. There might be a few more similar studies in steel out there ...
and that is "the rest of the story", as Paul Harvey would have said