Bilenky makes a tandem like that. I talked with a guy at the LBS who said he saw one couple doing the (oh Blast! What's that called?) tour from Alaska to Tierra del Fuego on one.

http://www.bilenky.com/viewpnt.html

It probably wouldn't pass your flat-foot test, though.

I've always wanted to try one, but have never had a chance. It's probably just as well, I'm not sure my marriage can survive a divorce cycle.