Since we’ve strayed from the OP’s original premise, that of two future stars appearing in an episode of a show I guess I can bring up this one (and I hope I haven’t brought it up on the boards before)
At the very end of the original version of “Psycho” when the officer is taking a blanket to Norman Bates, a young Ted Knight (Mary Tyler Moore Show, Caddyshack, Too Close For Comfort) is an EXTRA. All he does is open a door to let the officer in.
There was also one with Elizabeth Montgomery as David White’s* love interest.
*Aka Samantha Stephens and Larry Tate.
Why would it not count? That’s awesome.
We have? Why?
Anybody remember Holocaust? Meryl Streep and James Woods were an Aryan/Jewish couple; Michael Moriarty was the SS officer who investigated them. SFAIK, it was the big break for all three of them.
Joseph Bottoms and Tovah Feldschuh were also in it. I was madly in love with Tovah. :o
Ted Knight was also a German sergeant on Combat! Both Walter Koenig and Leonard Nimoy played GIs on the show, though not in the same episode.
Ok, This might be a bit off topic, but similar. This is a movie, not a TV episode. And the two actors hit it big in politics later on. Both becoming governors…
Predator
Combat! was particularly fertile ground for un- or little-known guest stars. Joseph Campanella and pre-Riddler Frank Gorshin were in an episode together. So were Jack Lord and Peter Duryea (aka Jose Tyler on the first Star Trek pilot).
Majel Barrett was also on The Untouchables, as an (uncredited) waitress.
Hawaii Five-0 had a two-parter with pre-MASH*** Loretta Swit and pre-St Elsewhere Ed Flanders.
Yeah but what I find odd about Knight being an extra on "Psycho" which came out in 1960, is that he’d already appeared with speaking roles on over six different TV shows, two years prior. A bit of come down from speaking parts on "The Twilight Zone" to opening a door on "Psycho".
And two actors hit it big in politics later on…
This Is the Army with Ronald Reagan (CA governor and US president) and George Murphy (CA senator).
Matthew Perry got his first acting credit in an episode of 240-Robert back in the '70s, presumably because he had the unfair advantage of being John Bennett Perry’s son; also getting work in that episode, Gerald McRaney.
Hmmm. I see your point, but I’m not sure going from TV to a major motion picture directed by Alfred Hitchcock is a “comedown,” even if he had no lines.
Having been in a number of productions myself, I can say the important thing is that you get noticed! Back in the '40s, there was one actor (whose name I don’t remember) who had one line as a sailor and was on screen for about five seconds; he became a national heartthrob overnight. Basically the same thing happened with Richard Dreyfus in The Graduate: “You want me to call the cops? I’ll call the cops!”
The Mary Tyler Moore Show had a lot of guest stars on their way up or down. One ep. had Laurence Luckenbill and Carole King, but both had kind of peaked fame-wise. One early episode, though, season 3 ep. 24, had a pre-fame Louise Lasser (Mary Hartman, Mary Hartman) and Craig T. Nelson (Coach).
All the shows she produced had a lot of improv comedians in them, from groups like Compass and The Committee, guys like Paul Sand and Roger Bowen. Except for Valerie Harper, appearing on an MTM show was a career peak for a lot of them.
This episode of the Philco-Goodyear Television Playhouse aired so many decades ago that Adam West (a) wasn’t even going by “Adam West” yet, and (b) was getting work alongside some nobody named Walter Matthau.
Pre-Star Trek William Shatner and pre-Batman Adam West together:
This one's a hoot and a half! :DNever mind…
These are all great replies! Thanks! I have a lot of things to check out tonight. I’m especially interested in finding that Young Indiana Jones with Daniel Craig and Katherine Zeta Jones, as well as the Dan August with Han and Lando. 
Wolfman, I would count all four of those in your A Team example. I admit, I had to look up Ogre and Leon, but instantly recognized both. And I love both Ernie Hudson and Michael Ironside as well!
I watched that episode of MASH last night and was very entertained. Even back then Andrew Dice Clay was a ham.
We can expand to small parts in movies as well if you’d like. I know for myself, I recently rewatched Scream 2 and was surprised to see a young Portia de Rossi and Timothy Olyphant in small parts.
Most Antipodean actors seem to have appeared in one of the daytime soaps Neighbours or rival Home and Away. Many have appeared in both.
Taking Neighbours in the mid to late 1980s. One of the stars was Alan Dale - already well known in Australia but yet to be a regular on The OC, NCIS, Lost, 24… One of his younger co-stars would be Guy Pearce who would find international fame in Memento, Prometheous and LA Confidential.
I don’t believe they shared any screen time together but joining Neighbours for a few episodes would be Guy’s LA Confidential co-star Russell Crowe before Gladiator, Master and Commander and Robin Hood.
Guy appeared regularly for years with Kylie Minogue. Perhaps best known now as a singer but she did also star in the film version of Mortal Kombat.
Much later (and not all at the same time) but all three Hemsworth brothers; Liam, Chris and Luke have appeared.
TCMF-2L