We've got a couple of tandems but our main ride is a 2006 steel, Co motion Speedster. (We'd love to upgrade, but to get anything even marginally better we'd really have to lay out the big bucks!)
As our daily rider, we use 40 spoke Dyads with 28mm Gatorskins. These are a solid set of wheels.
A few times a year (though not this year for obvious reasons) I throw on a set of Rolf Prima Vigor wheels, which I picked up on E-bay a couple of years ago. It turns out that these were an original upgrade for this bike if you purchased the 'race' package.
Back then 'narrow' = fast, so these rims are very narrow, 18mm outside, I think 13mm inside! I have 25mm Conti GP4000sll's on these babies. And according to some charts, that's pushing it. I think it was intended that you run 23's on them Twentythree millimeter tires on a tandem? I don't think I'm quite man enough for that!
Be that as it may, and back to the original question, this tire and wheel combination shaves over two pounds off the total weight of the bike, down to 32lbs. And they really seems to help the bike fly. It could be psychologically inspired, but we did average 20.6 mph for 54 miles with them on. On good days, with are daily set up, we are normally average around 17. That's a lot of psychology!