depending on how whatever tandem you find is made, you might be able to find two old front freewheeler sprockets and get independant pedaling. i have one on my freakbike tandem so the rear rider can coast. i got two for it really, but one had boogered threads. either person could coast then. i remember another person did this and he said they worked fine.
edit for pic: one thing though. i guess since these weren't really made to be part of a tandem drivetrain i get a lttle interferance bettween the chains if everythings not pretty tight. i guess on a real tandem sprocket there's a little more width bettween the sprockets.
