Found the transmission loss data I was thinking about. http://www.bikeforums.net/showthread...ion?highlight=

Forum poster asu_gt did testing the lab with power meters and got a 1.8% loss for the stoker (consistent with a single bike) and 6% loss for the Captain, although that loss dropped as low as 4.5% with both riders pedaling, indicating an efficiency loss overall for the tandem around 4%, compared to 2% for a single bike.

At time trialing speeds, that's going to be overwhelmed by the aerodynamic advantage.

As for the U.S. records I think they all come from Nationals, although I could be wrong. http://www.usacycling.org/forms/records.pdf they usually try to have as flat of course as possible to make times comparable.

Interestingly for the Elite male records, the single and tandem records are both held by John Frey. (with Rod Bush on the tandem)

So if you asssume Bush is about as strong as Frey that gives you a good idea of the relative tandem advantage.

On his single, Frey could "only" do 31.5 mph. On the tandem they managed 33.8mph, which is a pretty huge difference at those speeds.