“At the same time” can be sort of a vague concept with modern short streaming seasons with big gaps between them, but two “current” examples I noticed are Danny Pudi in both Mythic Quest and Going Dutch and Peyton List in both Cobra Kai and School Spirits.
Lisa Kudlow played Phoebe on Friends while also playing the ditzy waitress Ursula on Mad About You.
Kudrow.
Weren’t they canonically twin sisters?
Yes and no. Ursula (Phoebe’s less-nice but no-less-dippy identical twin) predated Phoebe by a year; Kudrow began playing the character on Mad About You in 1993 (its second season), while Friends premiered the next year.
And, while Phoebe was a lead character on Friends, Ursula was a lesser, recurring character on Mad About You, often only appearing in a scene or two in a given episode. Twelve of her 22 total appearances as Ursula on Mad About You were in 1993-94 (i.e., the year before Friends), and she only appeared on Mad About You five times after Friends’ first season (1994-95).
Goldie Hawn was still starring in Good Morning World when she first joined Laugh-In.
Jim Backus was in both Gilligan’s Island and the Mr. Magoo TV series. For a while, both shows were in the same time slot.
Not an actor, but Arthur Godfrey was in two shows at the same time in the 1950s.
Drew Carey and Ryan Stiles were both regulars on The Drew Carey Show while also appearing (as host and main cast member, respectively) on Whose Line Is It Anyway?.
*From a “Scenes from a Hat” segment on the latter:
DREW: “Charities no one donates to.”
RYAN: Help get Drew Carey a third show!
A good example: Richard Anderson and Martin E. Brooks, who simultaneously played Oscar Goldman and Dr. Rudy Wells, respectively, on both The Six Million Dollar Man and The Bionic Woman; other than Lee Majors (Austin) and Lindsay Wagner (Sommers), Goldman and Brooks were the only other two actors who appeared regularly on the shows.
Over the course of the 1976-77 and 1977-78 TV seasons, Goldman made 43 appearances on The Six Million Dollar Man, and 58 appearances on The Bionic Woman – every episode of both series in those two seasons; Brooks was also in all 43 of those 6MDM episodes, as well as 45 of the TBW episodes.
How about Glenn Howerton, who starred in A. P. Bio while It’s Always Sunny In Philadelphia was/is still ongoing?
Leo G Carroll as Alexander Waverly on both The Man and The Girl from UNCLE.
Just the example I was going to post
William Daniels was busily racking up Emmy nominations for Best Actor on ST ELSEWHERE while doing double duty on KNIGHT RIDER as primetime television’s answer to the question nobody asked: no, seriously: how condescending would an artificial intelligence sound?
David Hasselhoff was simultaneously on BAYWATCH and BAYWATCH NIGHTS.
Kenan Thompson had a one-season sitcom, KENAN, while also active with SNL.
Steven Culp had overlapping roles on THE WEST WING and STAR TREK ENTERPRISE, although he wasn’t in the primary cast of either.
Armin Shimerman pulled double duty on STAR TREK DS9 (regular cast member) and BUFFY THE VAMPIRE SLAYER (recurring character).
He was also part of the main cast of All That along with Kel Mitchell while both were also starring in Kenan & Kel.
Pedro Pascal was starring in The Mandalorian and The Last of Us at the same time, but the former only needed his voice most of the time.
Khandi Alexander was in Newsradio and ER at the same time, and I believe she left the former to focus on the latter when her character’s role was expanded.
If Always Sunny is still running, Kaitlin Olson is also appearing in a detective drama on ABC. Previously she was on a sitcom on Fox called The Mick. And Rob McElhenney also appears on Welcome to Wrexham.
Does voice acting count?
Mila Kunis was in That '70’s Show and also in Family Guy (doing a voice role).
Robert Reed. While he was on the Brady Bunch, he also appeared on Mannix as Mannix’s police contact. Mannix was a recurring role, he wasn’t in every episode. When Reed wasn’t there other actors would play similar cop characters. Reed’s appearances also tended to be early or late in the seasons for each show as he had a bit more time to do double duty then.
It also helped that Brady Bunch and Mannix were shot on the same lot. In fact, the Brady Bunch SET also did double duty, as it was used at various times for scenes in Mannix. Sometimes straight, and sometimes with things moved around, but the distinctive style was obvious.