More HOF Pitcher teammates who went on to other teams:
Lefty Grove and Waite Hoyt were briefly teammates.
Cy Young and John Clarkson both played for the Cleveland Spiders.
Christy Mathewson and Rube Marquard were teammates on the Giants; yes, Mathewson did play for another team, the Reds, albeit briefly. Then Marquard was a teammate of Burleigh Grimes on Brooklyn, and they both later moved on.
The individual champ has to be Gaylord Perry. Perry and Juan Marichal were teammates in San Francisco. Both moved on from there. Perry then played with Bert Blyleven in Texas, from where both moved on, and Perry went to San Diego and pitched with Rollie Fingers, and they both moved on, and then Perry played with Goose Gossage in New York, and they both moved on from there, and then he briefly went back to Texas and played with Fergie Jenkins, and they both moved on from there, and then he pitched with Phil Niekro in Atlanta, and they both moved on from there. Honestly, I may be missing a Perry connection or two.
Niekro, incidentally, was a teammate of Warren Spahn in his first tour of duty with the Braves, and it may surprise you to know Spahn at the end of his career pitched for the Mets and Giants so that qualifies.
I’m sure there are more but I’m doing this by hand. When Roger Clemens is finally put in the Hall of Fame he will qualify here a few times; he pitched with Roy Halladay in Toronto, and they both moved on.
What will be cool is if Andy Pettite also gets in; he and Clemens would actually count twice. They were teammates in New York, both went to Houston, and both went back to New York and were briefly teammates again.
If Curt Schilling ever gets in, and eventually I think he will once the stink of Trumpism goes away, he’ll also qualify, with Randy Johnson.