Just noticed a UK one, thanks to the proliferation of TV channels re-running old series:
The actors playing detective and pathologist in early series of Midsomer Murders (John Nettles and Barry Jackson) appeared together years before when Nettles played the Jersey policeman Bergerac and Jackson an annoying senior officer in Customs and Excise who takes over the office space.
Penny Marshall and Cindy Williams were first paired in an episode of Happy Days before they got their own spin-off series Laverne and Shirley.
Weren’t they playing Laverne and Shirley in both?
Yes, they were. But the spinoff lay in the future, after their first appearance together.
How is that different than Joanie Loves Chachi or dozens of other spinoffs? Maybe I misunderstood the OP.
Scoot McNairy and Kerry Bishé played husband and wife in the film Argo (2012) and then again in the series Halt and Catch Fire (2014-2017)
Happy Days and Laverne and Shirley were two different series. Initially, everyone (including myself) thought it was a one-off pairing.
Yeah, I don’t get it either. A spinoff is a spinoff.
Robert Morse and Rudy Vallee co-starred on Broadway in the musical How to Succeed in Business without really Trying in 1961, and repeated their roles in the 1967 film version.
Then they both appeared in a 1971 episode of Rod Serling’s Night Gallery entitled “Marmalade Wine”. Why they chose these two actors I do not know – it’s a pretty macabre turn for both of them.
Does Bea Arthur and Rue McClanahan from Maude and Golden Girls count?
If they appeared together in two different series at different times, I would say yes.
For that matter, Rue McClanahan and Betty White appeared together in a few episodes of Mama’s Family.
Rose Marie and Morey Amsterdam appeared on an episode of Caroline in the City, as an elderly married couple. Definitely not as their Dick Van Dyke Show characters, but that’s what they talked like of course.
Rue McClanahan was a one-off character on All in the Family, but did not appear with Bea Arthur, unfortunately for the thread; however, Betty White & Bea Arthur once appeared together on the Dinah Shore Show, singing “42nd Street.”
Jason Alexander and Julia Louis-Dreyfus played North’s parents in Rob Reiner’s North. Not sure if it counts for the OP because it came out during the middle of Seinfeld’s run.
Lucille Ball and Desi Arnaz’s first appeared together in the 1940 comedy/musical Too Many Girls, beginning their professional and personal relationship.
Jack Klugman and Tony Randall appeared together in an episode of Appointment with Adventure 15 years before The Odd Couple.
Mark Harmon was a recurring character on The West Wing when Lily Tomlin and Richard Schiff were primary characters (though I don’t recall him interacting with either of them). Both were later on NCIS with him. Gary Cole was in there too, but after Mark Harmon’s character was killed.
Interesting to me… Monk is my favorite show. In the first episode. Later I discovered Corner Gas, which is probably my favorite sitcom.
In the first episode of Monk two of the characters were played by Fred Ewanuick and Janet Wright. A couple years later they were in the main cast of Corner Gas.
(