I saw a Rigi when it was new in Huntington Beach, CA. I think it was at Carnevale's shop, but it might have been at Two Wheel Transit Authority. I was always fascinated by the Rigi, but never got to ride one. Actually I was afraid of it after reading the article in Bicycling.
The twin seat tube idea has been used by other bikes as well. In the thirties there was a British bike called a Saxon with twin seat tubes. There's an article about it on the Classic Bicycle site. Sorry, I don't have the URL handy.
Another twin seat tube bike was made by Puch for hill climbs in about 1981. They took weight savings to such an extreme that the fork crown had a hollow top and you could look down into the fork blades!
Ann Arbor, MI