I saw a couple years ago and even a soft-beam tandem but it has been years since. The tandem riders stopped using theirs when the beam developed a crack or some sort of structural problem they didn't want to deal with.

If you search around, you'll find that all the "latest and greatest" ideas are rehashed ones from decades or even over a century ago. New materials and manufacturing techniques breathe life into them, but most don't go very far.