Thanks for the suggestions. We checked the rim strip and inside of the tire...both were okay. I was wondering about the tire being damaged by the blowout, but we stopped almost right away when it happened. I guess that takes it to a problem with the initial installation or a failure of the tire...the tire was up to pressure when we started the ride.
The scary part is that a few minutes earlier, we'd been coasting down a hill and hit 30 mph...the fastest we'd gone yet (commented another rider, "Oh, you mean in that 25 zone?).
Anyhow, this sounds less like a tandem problem and more of a general biking problem.
Dan