In jazz, two pianos are usually one too many - but not when the second pair of hands belongs to Marian McPartland. As the host of National Public Radio's Piano Jazz, the longest-running jazz program in the history of network radio, McPartland has played more duets with more different pianists than anyone in the history of jazz, and the fruits of her vast experience can be heard on Just Friends, in which six of her favorite on-the-air partners join forces to celebrate her 80th birthday. -Terry Teachout