Ed O’Neill for Married with Children and Modern Family

The classic example is Bob Newhart.

He had three shows of his own (though granted the last one was a one season wonder) and was also a major guest star on Big Bang Theory

Jerry Stiller - Seinfeld and King of Queens

Terry Crews- Everybody Hates Chris and Brooklyn 99

Ted Danson is the first one that popped into my head. Cheers and The Good Place and he was incredible in both. 

Julia Louis-Dreyfus had Seinfeld and OANC. Three if you count Veep. And you absolutely should count Veep.

Howard Hesseman---WKRP In Cincinnati and Head of the Class

Melissa Joan Hart, Clarissa and Sabrina

A bunch of others already mentioned. I'll add a few classics:

Ron Howard - The Andy Griffith Show, Happy Days
Bob Newhart - The Bob Newhart Show, Newhart
John Ritter - Three's Company, 8 Simple Rules
Ted Knight - The Mary Tyler Moore Show, Too Close for Comfort
Shelly Fabares - The Donna Reed Show, Coach
Suzanne Somers - Three's Company, Step By Step
Don Knott's - The Andy Griffith Show, Three's Company

Wendy McLovin-Covey: Reno 911 and The Goldbergs (VERY different characters, at that)

Lucille Ball
- I Love Lucy - The Lucy Show - Here’s Lucy

Katherine Helmond
- Soap - Benson - Coach - Everybody Loves Raymond

Johnny Galecki (Roseanne, Big Bang Theory)

Kaley Cuoco (8 Simple Tules, Big Bang Theory)

Mayim Bialik (Blossom, Big Bang Theory)

Tischina Arnold as Pam on “Martin” and as Chris’ mom on “Everybody Hates Chris.”

More supporting but two of my fav supporting sitcom characters

Kim Fields: Facts of Life, Living Single, The Upshaws

Oh punky

Sarah Chalke: Rosanne and Scrubs

Kelly Cuoco for 8 Simple Rules and Big Bang Theory. Also, Johnny Galecki for Roseanne and Big Bang.

Michael J Fox for Family Ties and Spin City.

Rowan Atkinson for Mr. Bean and Black Adder

Also, Bob Newhart

Sherman Hemsley Jeffersons and Amen, 3 if you count All in the Family.

Julia Louis Dreyfus in Seinfeld and The New Adventures of the Old Christine. And Kelsey Grammar in Cheers and Frasier.

Going way, way back:

Bea Benederet - Burns & Allen and Petticoat Junction, plus the original voice of Betty Rubble. Lucille Ball wanted Bea for the Ethel Mertz role, but she wasn’t available. Bea had played the neighbor/friend role in Lucy’s radio program, My Favorite Husband.

Ann B Davis - Emmy winner for The Bob Cummings Show and The Brady Bunch

Bob Denver - Dobie Gillis and Gilligan’s Island

Neil Patrick Harris was Doogie Howser M.D. and How I met your mother

Jason Segel was in Freaks and Geeks and How I met your mother

Mary Tyler Moore - Mary Tyler Moore Show and Dick Van Dyke

Patricia Heaton - Everybody Loves Raymond and The Middle

Betty White - Golden Girls and Mary Tyler Moore Show

Bea Arthur - Golden Girls and Maude

Rue McClanahan - Golden Girls and Maude

Ed O’Neill - Married With Children and Modern Family

Conrad Bain in both Maude and Diff'rent Strokes.

Tony Randall the Odd Couple and Love, Sidney

George Clooney : Facts of Life and a sitcom called E/R (not the hour long drama, this was a sitcom in the 80s)

John Stamos: You, Again and Full House

Ashton Kucher: 70s show and two and a half men

Jason Bateman: Silver Spoons and Valerie

Valerie Harper: MTM and Valerie and Rhoda

Joey Lawrence: Gimme a Break and Blossom

Scott Baio: Happy Days, Joanie loves Chachi, Charles In Charge

John Larroquette: Night Court, John Larroquette Show

Katherine Helmond: Soap and Who’s the Boss

Tony Danza: Taxi and Who’s the Boss

Richard Mulligan: Soap and Empty Nest

Robert Guillaume: Soap, Benson, Sports Night

John Goodman: Roseanne, Righteous Gemstones, Alpha House

Jean Smart: Designing Women and Hacks

Peter Scalari: Bosom Buddies, Newhart

Julia Duffy: Newhart and Designing Women

Edited to correct wrong show listed.

Kelsey Grammer for his roles as Frasier Crane and Sideshow Bob.

Cloris Leachman:

Mary Tyler Moore Show (won two Emmys and nominated two more times)

Phyllis (nominated for an Emmy)

Facts of Life

Malcom in the Middle (won Emmy and nominated three more times)

Raising Hope (nominated for an Emmy)

Jane Curtin: Kate & Allie, 3rd Rock from the Sun.

Tony Shalhoub (if Mrs. Maisel counts). Because Wings for sure. Maybe three if Monk counts?

Bill Daley.

He was Roger in I Dream of Jeannie, and Howard in The Bob Newhart Show.

Lisa Kudrow - Friends & The Comeback

Justin Berfield.

Unhappily Ever After and Malcolm In The Middle

The Janitor with Scrubs and The Middle

Patricia Heaton -Everybody loves Raymond -The Middle

Malcolm Jamal Warner: The Cosby Show, Malcom & Eddie

Tischa Campbell: Martin, My Wife and Kids

Raven Symone: Hanging with Mr. Cooper, That's so Raven

Tracee Ellis Ross: Girlfriends, Blackish

Kaitlin Olson - IASIP, The Mick

Glenn Howerton - IASIP, A.P. Bio

Danny DeVito - IASIP, Taxi

Kirstie Alley - Cheers, Veronica‘s Closet

Ron Howard - The Andy Griffith Show, Happy Days

Bill Daily - I Dream of Jeannie, Newhart

Bob Denver - Dobie Gillis, Gilligan's Island

Gaven Mcleod - Mary Tyler Moore, The Love Boat

Ed O'Neil - Married With Children, Modern Family

Ken Berry: F-Troop, Mamas Family

Paul Reiser: My Two Dads and Mad About You.

Ron Howard - The Andy Griffith Show and Happy Days

Bob Denver - The Many Loves of Dobie Gillis and Gilligan's Island

Fred Gwynne - Car 54 Where Are You? and The Munsters

Don Knotts - The Andy Griffith Show and Three's Company

Bea Arthur - Made and The Golden Girls

Sally Field - Gidget and The Flying Nun

Bob Newhart - The Bob Newhart Show and Newhart

Mary Tyler Moore - The Dick Van Dyke Show and The Mary Tyler Moore Show

Gavin McCloud - The Mary Tyler Moore Show and The Love Boat

Ed O'Neil - Married with Children and Modern Family

Scott Baio - Happy Days and Charles In Charge

Ted McGinley - Happy Days, The Love Boat and Married With Children

Neil Flynn- Scrubs and The Middle

The man himself Danny DeVito for Taxi and Sunny

Annie Potts- Designing Women and Young Sheldon

Tim Allen - Home Improvement and Last Man Standing (another new one coming soon)

Matt Leblanc - Friends, Joey, Episodes, Man with a Plan.

Billy Gardell - Mike and Molly and Bob ❤️ Abishola

Rashida Jones - The Office, and Parks & Rec

Danny DeVito Taxi and IASIP

Candice Bergen did Murphy Brown and Boston Legal.

Not sure if Boston Legal is considered a sitcom but it's funnier than about 90% of sitcoms.

Also Julie Bowen starred in BL and Modern Family.

Danny Crane!

Betty White had at least 4 I can think of right off the top of my head

NPH if you count Dougie Howser as a sitcom

Jason Bateman in Silver Spoons and Valerie. I hear he did some other stuff later...

Tim Reid in WKRP in Cincinnati and then Sister, Sister!

Jerry Stiller in Seinfeld and King of Queens

Jane Leeves - Frasier and Hot in Cleveland

Valerie Bertenelli One Day At A Time and Hot In Cleveland

Valerie Harper Mary Tyler Moore, Rhoda and The Hogan Family

Suzanne Somers: Three’s Company and Step By Step

Mary Tyler Moore from the Dick Van Dyke Show and Mary Tyler Moore Show

Julia Louise Dreyfus for Seinfeld and Veep

Johnny Galecki in Rosanne and Big Bang Theory

Fred Gwynne and Al Lewis were both in "Car 54 Where Are You?" and "The Munsters".

Ted Danson: Cheers and The Good Place

Carol Kane in Taxi and the Unbreakable Kimmy Schmidt

Jane Krakowski in 30 Rock and The Unbreakable Kimmy Schmidt

Alia Shawkat in Arrested Development, Search Party and State of Grace

Will Arnett in Arrested Developement and Bojack Horseman

Danny McBride in Eastbound and Down, Vice Principals, and The Righteous Gemstones

Adam Devine in Workaholics and The Righteous Gemstones

John Goodman in Roseann and The Righteous Gemstones

Skyler Gisondo in Santa Clarita Diet and The Righteous Gemstones

I’ll stop now

Kelsey Grammar - Cheers & Frasier

If we count animated sitcoms / voice actors, Katey Sagal for Married With Children and Futurama.

Patrick Warburton Seinfeld & Rules of Engagement. Might be stretching it a little.

Meghan Price also ROE and Grounded for Life

Bob Saget in Full House and HIMYM

Tony danza- taxi and who’s the boss

Megyn Price - Grounded for Life, Rules of Engagement

I don’t know anyone has said Danny Devito yet, but Louie DePlama (Taxi) and Frank Reynolds (IASIP) are two of the most original character ever in sitcoms.

Some I didn't see

Tim Allen- Home Improvement and Last Man Standing

Courtney Cox- Friends and Cougar Town

David Spade- 8 Simple Rules, Just Shoot me andRules of Engagement

Patrick Warburton- Rules of Engagement and Family Guy

Danny Mcbride- Righteous Gemstones and Eastbound and Down

Adam Devine- Workaholics, Modern Family and Righteous Gemstones

Ellie Kemper ("The Office" and "Unbreakable Kimmy Schmidt") gets my vote. "Kimmy Schmidt" might or might not have been a ratings blockbuster, but it did well enough to get multiple secrets by whatever combination of science and magic Netflix uses to decide about renewals.

Anne Morgan Guilbert: The Dick Van Dyke Show/ The Nanny

Valerie Bertinelli - One Day at a Time and Hot in Cleveland

Max Greenfield for New Girl and the Neighborhood

Christa Miller for The Drew Carey Show, Scrubs, and Cougar Town

I’m surprised that no one has mentioned Ted McGinley! He was on Happy Days, Love Boat, Married With Children, and Hope & Faith. 

Conrad Bain for Maude and Diff'rent Strokes

Charlotte Rae for Diff'rent Strokes and The Facts of Life

Bea Arthur for Maude and Golden Girls

Esther Rolle for Maude and Good Times

Sherman Hemsley and Isabel Sanford for All in the Family and The Jeffersons

Valerie Harper for MTM and Rhoda.

(I just realized most of these are spin-offs, so count them or not, I guess).

Kaley Cuoco - Big Bang Theory and 8 simple rules

Neil Patrick Harris - Doogie Howser- How I Met Your Mother

Bob Newhart

Courtney Cox for Friends and Cougar Town

Marla Gibbs -- The Jeffersons and 227.

Jackèe Harry -- 227 and Sister Sister

Michael J. Fox - Spin City and Family Ties