Frank Oz was Bert, as well as Miss Piggy and Fozzie, and Henson was Ernie, as well as Kermit and a producer on the Muppet Show. I think they had enough to do on the show as it was.

Also: Oz was better at doing different voices than Henson. I think Ernie and Kermit in a scene together would be bad, since the voices were so similar.

Really makes sense. Thank you for your answer!

Aren't Ernie and Bert Sesame Street? IIRC, that's a different Muppet cohort (Big Bird, Grover, Oscar, etc.)

Kermit and Grover appeared in both the Muppet show and Sesame Street, there are intersections.

I don’t remember Grover ever being on the Muppet Show. Big Bird had a guest appearance once, perhaps that’s what you’re thinking.

Kermit was very much Jim Henson’s avatar and was the core character that the Muppet Show was built around, and existed before Sesame Street (IIRC, he was the very first puppet that Henson made, cut from his mother’s green coat). I believe that the Sesame Street Muppets other than Kermit are all owned by Children’s Television Workshop which is at least in part why they only made limited appearances in non-Sesame Street productions. There was a significant overlap in the puppeteering staff between Sesame Street and the Muppet Show (most notably Jim Henson and Frank Oz who were arguably the greatest comedy duo of the late twentieth century, but I think some other SS puppeteers also worked on the Muppet Show).

Of course, all of this is going from memory and someone going to Wikipedia could probably find all the places where I’ve misremembered things or am just plain wrong.

I remember going to a muppet exhibit at what is now the museum of popular culture in Seattle and finding out that the muppet Henson most identified with was Rowlf.

I also learned that Henson did some delightfully bizarre commercials. My favorite was this one for La Choy chow main. It absolutely caused me to buy some: https://youtu.be/4bfdaR4xMeU

Grover too? To me, they're almost entirely separate. Kermit is the only one I remember who regularly appeared in both. I think there's a very brief Big Bird cameo in the Muppet Movie, but I cannot recall any other significant character from one show appearing as more than a brief cameo in the other.

The Muppet Family Christmas has them all, even Fraggles Rock. We started watching every year thanks to a comment on HN. Somehow I’d missed it.

I still watch this every year. I first saw it on TV in 1987 and taped it, wearing that tape out. Nowadays, it's on YouTube.