There are a number of actors from Slow Horses who all worked together on Wolf Hall: Aimee-Ffion Edwards; Jack Lowden, Jonathan Pryce, and Saskia Reeves. I might have missed some.
Eugene Levy and Catherine O’Hara have been in a bunch of stuff together, starting back at SCTV in the late 70s. I absolutely adored Schitt’s Creek; they are both brilliant in it.
John Banner and Werner Klemperer, of “Hogan’s Heroes” appeared together on “Alfred Hitchcock Presents” S1 E22 “Safe Conduct.”
Allison Janney and Bradley Whitford worked together on The West Wing and now on The Diplomat.
Played by Karl Urban as I recall.
How did she escape that one with her head?
Getting off-topic, but if anyone wants a Xena thread, I’d be happy to start one.
Smuggler’s Bay, a 6-part adaptation of the classic adventure novel Moonfleet starred Frazier Hines and Patrick Troughton.
Years later they would co-star as Doctor Who and Jamie.
Allyson Hannigan and Seth Green (Willow and Oz on Buffy) were both in My Stepmother Is An Alien (and in fact, their characters went on a precocious date together)
Except his name is Jon Cryer, not Cypher.
Somewhat the reverse of what the OP is looking for: actors who appeared in an earlier series, and later appeared in a work.
Dick Van Dyke and Mary Tyler Moore played Rob and Laura Petrie for five seasons on The Dick Van Dyke Show. Years later, though the roles were different, Van Dyke and Moore would work together again, on a one-shot PBS production of The Gin Game. It’s quite good, and it is apparent that the chemistry they had before was still there.
Benedict Cumberbatch and Martin Freeman were the leads in Sherlock. They were also having Hobbit on Dragon conversations in The Hobbit movie.
Christopher Guest, Michael McKean, and Harry Shearer in Spinal Tap from 1984 reunited as the Folksmen in The Mighty Wind from 2003.
Most of the Christopher Guest mockumentaries featured a lot of the same actors, so a million examples there.
They also worked together in The Mary Tyler Moore Hour, an attempt to fuse a variety show and sitcom. The writers were trying to figure out skits for Van Dyke and decided that since Mary looked a lot like the actress who played Laura Petrie, they came up with a series of skits about Rob and Laura.
The topper was the end. Mary had been delayed and finally meets Dick van Dyke. She mentions that she auditioned for the Dick Van Dyke Show, but Rose Marie got the part.
I realized my earlier example doesn’t fit the OP at all, so I’ve been racking my brain for a way to atone. All I’ve come up with is a reverse example: Jane Curtin and Susan Saint James co-starred in How to Beat the High Cost of Living a few years before the series Kate & Allie.
Sorry, it’s the best I can do.
Ted Shackleford and Joan Van Ark played Gary and Val Ewing on Knots Landing, a spin-off of Dallas, starting in 1979.
About a year earlier, in 1978, they had both appeared in an episode of Wonder Woman, playing time travelers who visited from the 22nd century, one good (Shackleford) and one evil (Van Ark).
David Wayne and Larry Hagman, “Digger” Barnes and JR Ewing on Dallas, were previously in the ‘70s sitcom The Good Life (not to be confused with the British series of the same name starring Ricard Brier and Felicity Kendall).
Donna Mills (Abby Cunningham on Knot’s Landing) played Hagman’s wife on The Good Life and would appear alongside him again in at least one Knot’s Landing–Dallas crossover episode. (IIRC, the two ended up in bed together.)
Tim Conway and Joe Flynn of McHale’s Navy fame were reunited in the short-lived sitcom The Tim Conway Show (1970).
Not a TV show, but the 1972 Peter Bogdanovich movie What’s Up, Doc? had Madeline Kahn, Kenneth Mars, and Liam Dunn two years before they all appeared together in Mel Brooks’ Young Frankenstein.
The 1921 film The Lucky Dog featured both Stan Laurel and Oliver Hardy before they became a team six years later.
Steve Martin and Martin Short were both in Three Amigos! and a couple Father of the Bride films decades before Only Murders in the Building.