John Darling and Crankshaft used to spill over into Funky Winkerbean kind of a lot.
Several years ago, the Crocodiles babysat for the Baby Blues kids, who stayed off camera in PBS. The following Sunday’s strip for BB had Baby Wren in the background, chewing on a dead croc’s tail.
Some individual pairs of artists have done it but nothing ever on that scale was ever done again. It even crossed over different syndicates. At some point I remember seeing a website that had reproductions of most of the strips from that day but it was missing some. Nearly every strip printed at the time took part.
Meh, could be weirder.
As I recall, there was talk of doing it again the next year, but the bigger-name cartoonists balked and no one gives a crap about the lesser-known ones, so they stopped.
There were some instances of cartoonists using the occasion to either disrespect the strip they were subbing on (the Bizarro guy on Luann comes to mind) or using someone else’s characters to make dodgy political points. Anyway, no, it was a one-and-done.
Thank you for your contribution. I will treasure it forever.
And then there’s the time Bill Watterson drew Pearls Before Swine.
This may be of interest. I found it while trying to discover what Bizarro did to Luann.
There was a Peanuts strip that featured Charlie Brown and Alfred E. Neuman.
The February 14, 2000 strip of For Better or For Worse takes the form of a Happy Valentine wish to Sparky (Charles) and Jeannie Schultz, including drawings of Peanuts characters.
Well, it struck *me *as weird since Charles has died on the 12th.
Speaking of Peanuts, Weapon Brown features a weaponized Charlie Brown and some other comics characters.
Bizarre.
I don’t think that spinoffs really count as crossovers.
You can see that here, 'bout ¾ down.
Thanks. 
I love this recent B.C. strip
I tried to find an image of it online, but anyway…an old continuing story from “Bloom County” back in the day had Opus trying to recover from amnesia. (I don’t remember just why he had amnesia, but he had it for several weeks or months worth of strips.) Milo and Binkley are discussing it and the latter tells the former that he sent Opus to seek “psychiatric advice.”
The last panel of the strip depicted a befuddled Opus sitting on the stool in front of Lucy’s psychiatric advice booth from “Peanuts”, with the sign saying “the doctor is *out”.
In the next day’s strip, there was a panel just depicting the booth itself and a sign reading “the doctor is not seeing any more stupid penguins.”
Here’s a blog post by Dan Piraro about it. I think he did well with it, but then, I was a Bizarro fan.
Alfred also made a cameo in a TV episode of Mr. Magoo. Seriously, Magoo was Jack in a “Jack and the Beanstalk” episode, and at the very end, the giant turns out to be Alfred E. Neuman.
Now that you mention it, I remember that cartoon.