Apologies if these have been posted before. Found a guy on Youtube with an extensive collection of Chicago bootlegs. He's got good soundboard recordings of the Beach Boys / Chicago tour from 1989. Here's the songs both bands performed together on stage:
"Darlin" - Carl Wilson sings the first verse and Jason Scheff sings the second.
http://www.youtube.com/watch?v=aa7uNH3Am1g"You're The Inspiration" - The Beach Boys add extra harmony on the chorus. Jason Scheff sings the first verse. Carl Wilson comes in at the second verse.
http://www.youtube.com/watch?v=nbAD2LkTRE8"Wishin' You Were Here" - I'm sure everyone is aware that the Beach Boys (well...Carl, Al and Dennis at least) originally sang backup vocals on the studio version. On this live performance Robert Lamm sings the first verse, Mike Love takes care of the second, and Carl comes in at the bridge and proceeds to sing the hell out of it!
http://www.youtube.com/watch?v=lIZKUCLShvQ"Help Me Rhonda" - That's Al Jardine singing the first verse and Bill Champlin on the second. Only Bill Champlin could take a "gee whiz" song like this one and turn it into a bluesy soul-fest. Horns provided by Chicago. Crazy piano solo most likely provided by Champlin.
http://www.youtube.com/watch?v=OWrT8eSLX3k"Does Anybody Really Know What Time It Is?" - Al Jardine provides the vocal on the first verse (the second verse is oddly enough skipped) and Robert Lamm completes the final verse.
http://www.youtube.com/watch?v=5T5Lp-pcd-g