"Fran & Marlo Cowan (married 62 years) playing impromptu recital together in the atrium of the Mayo Clinic. He'll be 90 in February."Found at Within the Crainium.
Addendum: The piece is “Put on Your Old Gray Bonnet.” More about the couple at MinnPost.
Updated from 2009 to add some new links. After that spontaneous duet captured on the first video, the couple were invited back to the Mayo Clinic for other performances.
This video in 2010 compiles several of their routines.
This 2011 video covers their "final performance."
It's "old-time fun."