the other goes on to do ok(or worse) for him/herself.
The classic case is Tom Hanks and Peter Scolari of Bosom Buddies. Also Tom Hanks and Adrian Zmed of Bachelor Party.
Keanu Reeves and Alex Winter of Bill and Ted fame.
Anthony Michael Hall and Ilan Mitchell-Smith of Weird Science
George Michael and Andrew Ridgeley of Wham!
Basically people who got their big break together but one went on to outshine the other through talent and/or hype. Movies, TV, music, etc.
Ike & Tina Turner?
Paul Simon & Art Garfunkle?
Tom Hanks and Peter Scolari were the duo that first came to mind when I saw the thread title.
There’s also Penny Marshall and Cindy Williams, Laverne & Shirley. Marshall has gone on to success as a director. Cindy … um, I have no idea.
Neil Mullarkey and Mike Myers.
Sonny and Cher.
Stephen Fry and Hugh Laurie did a few shows together. Fry isn’t exactly a nobody these days, but he’s no House.
Al Franken has outfamed Tom Davis.
I’d say that Fry is still bigger than Laurie in the UK (And no, I don’t just mean physically, although the man *is *gargantuan). I’d call them the rare example of a duo in which both members went on to greater glory.
Penn and Teller.
Penn Gillete has a big career and Teller hasn’t been heard from. 
Loggins & Messina.
Melanie Lynskey & Kate Winslet from Heavenly Creatures.
The former was by far the better actor but…not as pretty. And so here we are.
ETA: Or…perhaps she bloomed late. Rowr.
Naomi Watts and Laura Harring from Mulholland Dr.
Pierce Brosnan and Stephanie Zimbalist from Remington Steele.
I’m surprised no one mentioned Harrison Ford and Mark Hamill from Star Wars yet, as they’re probably the most well known example of this. One went on to become one of the most famous actors of all time, and the other didn’t do so well.
Steven Fry is about as big as Laurie if not bigger. In addition to being an actor he is a major TV personality and writer. He is also a big-time blogger/Twitterer and something of a tech guru.
Scarlet Johanssen and Thora Birch in Ghost World. Not exactly a duo but their relative career paths roughly mirror the story of their characters in the film.
Sonny & Cher
Mike Myers and Dana Carvey from Wayne’s World.
As I recall Carvey was supposed to be the big breakout star from his class of SNL alums, but sort of just disappeared in the mid-90s while Myers went on to star in the Austin Powers and Shrek films.
Jack Black and Kyle Gass was the first to come to mind.
Gervais and Merchant
And in related news, Cybil Shepard and Bruce Willis from Moonlighting.
(Stupid brain! You can’t remember the names of your friends’ children, but you can remember this?!)
A non-example that I just thought of was Naomi Watts and Nicole Kidman. The two of them went to high school together and both starred in Flirting. Obviously they both made it in the end, but it took Naomi 11 more years.
I’ve noticed her as the nice “homely/cute” step-sister in EVER AFTER and the recurring character Rose in Two and a Half Men.
Melanie Lynskey went out with a friend of mine for a while. I only found this out after they’d broken up, so never got to meet her, worst luck. I’d always wanted to, as, I agree, she was far better in Heavenly Creatures than Kate Winslet (though, to her credit, she has gone on to prove her talent).
