......

Yes the connection was “Mad About You”

Kramer (Seinfeld) and Ursula Buffay (Phoebe’s evil twin - Friends) had cameos on “Mad About You”.

Funny enough, I just watched a YouTube video about “TV crossovers people forgot about” a few days ago. And “Mad About You” was referenced.

None whatsoever.
But there was between Friends and Mad About You (Phoebe and Ursula are twin sisters)
Edit : Ninja'ed

They both took place in New York and were both #1 sitcoms on Must See TV on NBC. That's about it.

I wonder if the particular actors you referenced just had agents with connections at NBC.

Was there a connection between Friends and ER, though? Did Dr Ross and Dr Carter treat Monica, or were they just two doctors who looked a lot like them?

The New York doctors Dr. Mitchell and Dr. Rosen just looked like the two doctors from Chicago.

Well, if Kramer was subletting Paul Buchman's old apartment on Mad About You, and Ursula was on Mad About You, and Ursula was also on Friends, then Seinfeld and Friends exist in the same fictional NYC.

And David Schwimmer played Ross on an episode of The Single Guy!

But George watches Mad About You on TV.

NBC wasn’t a network, it was a recursive dream loop in which the same like two dozen actors wandered an eternal Manhattan, laughing, dating, aging out of relevance, and being reabsorbed.

No one escaped. Not Jon Lovitz. Not the Silverman douchebag. Not dignity.

We watched slack-jawed as time folded in on itself whilst Courtney and her magic Cox baked cookies for men who would never love her.

The laugh track wasn't for us. It was for them. They needed to believe something was funny. God help them. God help us all.

Paul Reiser is Schrödinger’s landlord.
Phoebe is her own evil twin.
Time is a soundstage in Burbank.
And Baba Booey to us all.

Seinfeld exists in our world. Mad About You is a TV show they watch. Jerry was interviewed on The Today Show. Kramer got cast in a role on Murphy Brown — but he doesn’t share a reality with Murphy Brown, only with Candice Bergen.

Seinfeld the TV series does not exist in our world because in our world Jerry Seinfeld the comedian exists and is a different person.

And it may have just been a cheap and tacky promotional stunt on NBC's part, but Kramer and Jerry the character share a reality with Mad About You.

Do they play the same characters on both shows?

The obvious answer for so many guest actors appearing on both shows would be that both shows had the same casting company, but they did not.

So maybe it was just laziness.

I had no idea Kramer was on Mad About You (because I never watched MAY).
So I guess that answers the original question as someone already mentioned... same universe... Seinfeld to Friends via the Kramer on MAY to Phoebe's sister to Phoebe connection.

And it has to be the same characters that appear on the same show - not the same actors. Otherwise almost every show is in the same universe.
Hell, anyone ever notice how many actors have been on both Parks and Rec and Brooklyn 99? I can probably pick out at least 25 who've been on both.

Both Col. Klink and Lurch (in character) appeared on Batman.

The Col Klink appearance really blows my mind since his show was set 20 years before Batman, and HE'S A NAZI

Pretty much every tv show takes place in the mind of an autistic child...

https://www.unsupervisednerds.com/re...phall-universe

https://thetommywestphall.wordpress....e-master-list/

I've always been fascinated by the fact that someone actually pitched a sitcom set in a Nazi prison camp with the war still fresh in memory, and someone else said "Great idea! Let's make it happen!"

The Cheers / St Elsewhere crossover was the weirdest one imaginable. Didn't watch The White Shadow so I never realized that a character crossed over.

FWIW : I hated the ending of St Elsewhere. Didn't think it worked at all or made any sort of logical sense. To this day I think it continues to hurt the legacy of that show.

People weren't so woke back then

Romy and Michele's High School Reunion is part of TarantinoVerse.

I'm starting to think that these producers and writers are just making shit up as they go along without considering the larger metaphysical implications of their "little jokes."

War time comedies were common. It's just a comedy version of Stalag 17 or The Great Escape. McHale's Navy is comedy about war in Pacific.