Here are some songs where the singer or group was backed up vocally by another famous singer or group. The B/As are in parentheses. Whenever possible, I'm excluding songs which are considered duos or duets.
"Buried Treasure" - Kenny Rogers (Larry Gatlin)
"Same Old Me" - George Jones (Oak Ridge Boys)
"Rambling Man" - Bob Seger (Glenn Frey was in the backup band at the time)
"Love Has Come" - Amy Grant (Bill Champlin)
"Love Affairs" - Michael Martin Murphey (Jennifer Warnes)
"Part Time Lover" - Stevie Wonder (Luther Vandross)
"Twenty Years Ago" - Kenny Rogers (Bill Champlin)
"Long Tall Texan" - Doug Supernaw (Beach Boys)
"Wishing You Were Here" - Chicago (Beach Boys)
"Don't Forget Me (When I'm Gone)" - Glass Tiger (Bryan Adams)
"Money For Nothing" - Dire Straits (Sting)
"Running With the Night" - Lionel Richie (Richard Marx)
"Under Pressure" - Queen (David Bowie)
"It Used to Be Her Town" - James Taylor (J. D. Souther)
"Honky Tonk Stomp" - Brooks & Dunn (Billy Gibbons)
"Rockstar" - Nickelback (Billy Gibbons)
"Buried Treasure" - Kenny Rogers (Larry Gatlin)
"Same Old Me" - George Jones (Oak Ridge Boys)
"Rambling Man" - Bob Seger (Glenn Frey was in the backup band at the time)
"Love Has Come" - Amy Grant (Bill Champlin)
"Love Affairs" - Michael Martin Murphey (Jennifer Warnes)
"Part Time Lover" - Stevie Wonder (Luther Vandross)
"Twenty Years Ago" - Kenny Rogers (Bill Champlin)
"Long Tall Texan" - Doug Supernaw (Beach Boys)
"Wishing You Were Here" - Chicago (Beach Boys)
"Don't Forget Me (When I'm Gone)" - Glass Tiger (Bryan Adams)
"Money For Nothing" - Dire Straits (Sting)
"Running With the Night" - Lionel Richie (Richard Marx)
"Under Pressure" - Queen (David Bowie)
"It Used to Be Her Town" - James Taylor (J. D. Souther)
"Honky Tonk Stomp" - Brooks & Dunn (Billy Gibbons)
"Rockstar" - Nickelback (Billy Gibbons)