McCartney will join Kimmel in his Los Angeles television studio for an interview Monday, then perform a free concert outside for the waiting masses.
The following night, Timberlake will sing songs from his new album, "The 20/20 Experience -- 2 of 2," which hits stores Sept. 30. Timberlake will also chat with Kimmel about his role in the new movie "Runner Runner," which opens in theaters Oct. 4.
"This unprecedented two-night affair is a free concert experience in the heart of Hollywood," ABC said in a release Tuesday. "Streets will be closed and traffic will be diverted in an effort to contain the anticipated crowds. 'JKL' will close down a quarter-mile of Hollywood Boulevard from Highland Avenue to Sycamore Avenue in order to accommodate as many fans as possible."