Laverne and Shirley was a Happy Days spinoff.
It wasn’t that seamless, what with Bea Benadaret playing Jethro’s ma on the Beverly Hillbillies, but Kate on Petticoat Junction.
The in-universe explanation was that Kate and Pearl were cousins. Hey, it worked with Patty and Cathy Lane!
there was a 70s version of a plastic man cartoon where he had a blonde dim wit girlfriend (whom he married and had a baby with) and a side kick who was called something like “hula joe” because he was Hawaiian but all he did was sound like a whiny Costello and mess things up
Since off-topic comic books have already been mentioned…did you see the Punisher/Archie crossover? I never read it, but heard it worked surprisingly well (at least relative to the absolute disaster one would think it would be).
So was Mork and Mindy. IIRC, Mork, Laverne, and Shirley were in the same episode of Happy Days.
Laverne and Shirley debuted a couple of weeks before I left to live abroad. I thought it was hilarious! I came back 14 months later, and found it had devolved into a Lucy Show clone. 
Was that the giant computer that was supposed to run everything in the world, but malfunctioned (due to the mouse, I’d guess) badly requiring the Superfriends to disable it? I saw that one, back in the day.
Oops, I see now that M&M and*** L&S*** were dealt with on Page 2.
Never mind. 
Sounds about right. Those early episodes were pretty goofy. Things didn’t get exciting until the Legion of Doom showed up!
OK, this technically isn’t a crossover, but in 1990 there was an animated tv special called Cartoon All-Stars to the Rescue warning about the dangers of drug use. It featured characters from the following:
ALF: The Animated Series
Alvin and the Chipmunks
DuckTales
Garfield and Friends
Looney Tunes and Merrie Melodies
Muppet Babies
The New Adventures of Winnie the Pooh
The Real Ghostbusters
The Smurfs:
Teenage Mutant Ninja Turtles
It was horrendous.
https://en.wikipedia.org/wiki/Cartoon_All-Stars_to_the_Rescue