This might be pushing, but I remember when Davie Jones of the Monkees visited Marcia on the Brady Bunch-Monkees Show may have been history by then though.
Here's one...When "Adam 12" cast would team up with cast of "Emergency!"
Who was the comedian who popped up on Lucy, Sanford & Son, Flintstones & MANY other shows with snide remarks. He'd always turn around and say "YYYYYEEEEESSSSSSSSSSSSSSSS?


"
I remember after "The Ropers" got their own show they went back & visited Jack, Janet & Cindy.
Didn't the real Carol Burnett go on Mama's Family once?
Alf filled in for Carson one night!
Baywatch & Baywatch Nights cast used to jump back & forth sometimes.
C.H.I.P.S had an all star rollerskate episode but all I can remember is Erve Velechez(Tattoo) on skates & Ponch & John pulling over H.R PuffnStuff on the freeway but that wasn't really "cast visiting cast"-it was "actor visiting show".
Joanie Loves Chachi always had to have old Happy Days cast visit to survive the short stint they did!
Xena Warrior Princess popped onto Hercules the Legendary Journeys & vise-versa a couple times.
The Bionic Woman & Six Million Dollar Man visited each other sometimes.
...more to come!