I dunno about that. Cher’s performing career was better, but Sonny got himself elected to Congress, and had this major copyright bill named in his honor, posthumously.
It does say he was diagnosed in 1984 and it would explain why he did more voice over work (wow, lots and lots of voice over work, I had no idea!) and guest appearances. Back then treatments to control the effects of MS sucked, and I expect it would have been quite a career crusher.
I’d argue that each is exactly as successful as the other, within their specialty.
Made me laugh. 
Peter Cook and Dudley Moore.
That’s infamy, not superstardom. 
True, but they shared the fame pretty much equally for Jesus Christ Superstar. In 1971, it wasn’t like anyone knew who just one of them was, but not the other.
Early in his career George Carlin worked with Jack Burns. Carlin became famous and while Burns did have a good career, it was not on the level as Carlin’s
Peter Asher had more post-singing success as a record producer for Linda Ronstadt than his partner Gordon Waller. Of course what success Peter and Gordon had was because Asher’s sister Jane dated Paul McCartney and he wrote “A World Without Love” for them.
Yes, they share fame equally as a composer and lyricist respectively. If you asked me today, “name a famous lyricist” Rice would probably be the first name out of my mouth. It’s like what Face Intentionally Left Blank said, they are equally famous in their respective careers.
It’s not the same thing as Tom Hanks and Peter Scolari, or Keanu Reeves and Alex Winter, who shared equal billing doing the exact same thing.
Teresa Ganzel and Jim Carrey were paired together as a romantic couple in the TV show The Duck Factory. One went on to fame.
It would be more unusual to find a duo where both members achieved superstardom after their break up… One of the things that always amazed me about the Beatles, for example, is that two members stayed major stars after the breakup, a third was very successful, and even the least successful of the four (Ringo) still did extremely well for himself, and is still a household name 40 years later.
Telma Hopkins and Joyce Vincent Wilson were the two singers in Dawn (as in Tony Orlando and). Hopkins then went on to appear in several sitcoms but Wilson pretty much remained a backup singer.
It’s a little unfair to compare Mark Hamill to Harrison Ford. Ford was already an established actor by the time Star Wars was filmed. George Lucas wanted to cast complete unknowns in every role. Ford was just helping out by doing readings with the auditioners. During the auditions, Lucas realized that Ford was the best choice available to play Han Solo.
How has Carrie Fischer fared?
Then again, I heard that Alec Guinness did OK for himself.
Not a duo, certainly, but both Peter Gabriel AND Genesis were far more successful after they broke up than before.
Both Dean Martin and Jerry Lewis remained big stars for decades after they split up.
I actually remember that show, but I don’t remember them as a couple. I thought Ganzel was the ditzy, widowed trophy wife of the boss who died in the first episode (or something like that).
Wow, looking up her page on Wiki I’m surprised at how busy she’s been!
I mean, I knew she was a best selling author and all, but I didn’t realize she’d one so much guest starring stints. I’ve always had the impression she’s does those for the fun of it, not as serious career stuff, with writing being the real meat-and-potatoes of her career.
Ford may have been working as an actor but he still pretty much an unknown. Most of his roles had been TV guest shots - the equivalent of playing the suspect on an episode of CSI or Law & Order. His two biggest movies had been American Graffitti and The Conversation - but he had very small parts in both of those. Han Solo was definitely a breakthrough role for him.
I remember a Tom Snyder “Tomorrow” show in the mid 70s when he had on Arthur “Son of Groucho” Marx who had written a book on Martin and Lewis. Snyder remarked when the duo broke up, everyone assumed Lewis would be the big star. Martin was seen as a boozing not very talented singer. While Lewis had success, Martin had the bigger career.
To a lot of people outside country music, Dolly Parton is more well known than Porter
Waggoneer.
I would credit Martin’s TV show for keeping him in the public eye long after he stopped being a marquee headliner. He was canny enough to see the writing on the wall, as Alec Baldwin and Jeff Goldblum currently are doing.
How about a trio? Who in 1975 (Rocky Horror) would you have picked as a Hollywood survivor 35 years on: Tim Curry, Barry Bostwick or Susan Sarandon?
Another pair to consider: Ron Howard and Henry Winkler.
The way I always heard was that Harrison Ford had practically given up on acting because he couldn’t get enough work to support himself. He was on the set of SW as a carpenter when Lucas asked him to help with the readings.
It may not fit into the OP of “duo” but I’m sure when the rock group Cream broke up in the 1960s, people figured Jack Bruce would have the most successful career. He was the main singer and songwriter with lyricist Pete Brown (although Ginger Baker says Bruce took songwriting credit/royalties for arrangements he did for “white Room” and others). Eric Clapton sang a few songs but was thought to have a weak voice and shy stage presence. But Clapton instead helped create the era of the Rock Guitar God and improved his singing and songwriting.
Bruce’s prickly personality was a detriment in his career.
I think in recent years David McCullum has improved his public profile but didn’t he once guest on an episode of “Babylon 5” and wasn’t listed in the credits? Robert Vaughn, his partner in “Man from UNCLE” tended to have more roles (although originally Illya Kurachin was a minor character, fan mail increased his presence).
Short life but Bruce Lee became a larger figure than his “Green Hornet” series star Van Williams.