In Astérix and the Vikings, Asterix and Obelix take this role, with Justforkix as the main character. This is particularly silly, since Asterix's name is in the title of the film, and Justforkix was only a (to be fair, strong and memorable) side character in Asterix and the Normans, the comic the movie is adapted from. One reviewer on That Guy with the Glasses complained it was extremely strange since Asterix and Obelix are better-written, better-animated and designed, and easily the most interesting and likeable characters in the film, yet are relegated to backup for a whiny brat who gets no comeuppance.
Joey, Jeffy, and Jamie from Daria. They pretty much exist just to remind us that Quinn is attractive when she doesn't have a Boy Of The Week around to do it instead. In fact, they're SO Those Two Guys, Jamie is usually called something else, only to fruitlessly repeat that his name is Jamie.
Brian and Stewie have played this role in episodes where they are the focus.
Cleveland and Quagmire fit Those Two Guys until Quagmire started getting his own episodes and Cleveland got his own show.
Tomik and Belgarde, the two blond guys from an unspecified region of Europe "who have been living in the United States almost long enough to sound American." It's is not clear who is Tomik and who is Belgarde, and it probably doesn't make a big difference.
Officers Smitty and URL in Futurama. They are both police cops who were partnered up with each other.
Mr. Black and Mr. White. They were revealed to be federal agents who are also mutants, but they have not used any of their powers in future episodes.
Mertle Edmonds' posse in the Lilo & Stitch franchise, who mock Lilo with a drawn-out, sarcastic "Yeah!" Although they've been around since the original film, their names (Elena, Teresa and Yuki) weren't revealed until Lilo & Stitch: The Series.
The Lion Guard has Kiara's two friends, Tiifu and Zuri. The two are always seen together, usually with Kiara around as well.
Youngmee, Sue, and Jasper in Littlest Pet Shop (2012). Of these three, Youngmee gets preferential treatment by the writers by virtue of being neighbors with Blythe, the protagonist. And Youngmee becomes the only other human to know Blythe's ability to talk to animals.
It introduces Snips and Snails, a pair of excitable boy unicorns, in the episode "Boast Busters".
Lyra and Bon Bon were put side-by-side in the background often since their color-schemes complimented each other. The fans noticed and they became this thanks to a combination of Ascended Fanon and Ho Yay.
Other background pony pairings are common enough for fans to see them as couples, for example Time Turner/Doctor Hooves x Roseluck, Golden Harvest/Carrot Top x Written Script, or Carrot Top x Derpy/Muffins.
Twilight Sparkle and Spike have this chemistry at times, usually in episodes where they are supporting characters.
The show also occasionally likes to pair certain ponies together for particular episodes, even if these examples aren't nearly as consistent due to spotlight rotation. Rainbow Dash seems to form a good few of them (usually with Applejack or Fluttershy).
Baljeet and Buford have become this quite quickly, with an interesting Friendly Enemy dynamic as well.
Major Monogram and Carl.
Sometimes, in episodes that focus on Candace, Perry or Doofenshmirtz, Phineas & Ferb themselves get demoted to this.
Two guys that remain minor from beginning to end on the show are the nameless husband and wife. They're in several episodes for the sole purpose of the same Running Gag. The wife will say, "I can't believe you bought a [fill-in-the-blank product] [fill-in-the-blank product rental/selling place] that didn't have any [fill-in-the-blank products]. What, did you think [fill-in-the-blank products] would just fall from the sky?" Then, the [fill-in-the-blank product] will indeed fall from the sky, usually landing on top of the wife.
Token as he's oftentimes seen with Clyde and Craig. Plus the things that make him stand out (being rich and being black) are purely superficial, and he is otherwise the most normal kid.
Steven Universe has Lars and Sadie, the pair of (not entirely platonic) Vitriolic Best Buds who run the Big Donut. Even outside of work, they are seen together most of the time. When Lars gets left behind on Homeworld, Sadie becomes angry and depressed, even quitting her job at the Big Donut because she feels so lonely working there without Lars.
In the Teen Titans episode "Things Change", the girl who may or may not be Terra is flanked by two unnamed schoolgirls who seem to be her friends.
Several such pairs exist in Transformers Animated: Blitzwing and Lugnut, Bumblebee and Bulkhead, and Mixmaster and Scrapper, and Snarl and Swoop. In the Transformers comics, Rack'n'Ruin of the Wreckers, who were bonded together in a desperate attempt to save their lives. The fact that they basically have one name (and it's never specified which is which), and that they receive basically zero characterization even as a pair makes them a perfect example.
Pete White and Master Billy Quizboy from The Venture Bros., who are also business partners and roommates in addition to being super-scientists. In fact, most of the series' characters are foils for one another; Doctor Venture and Brock, The Monarch and Dr. Girlfriend, Hank and Dean, Watch and Ward, Monarch Henchmen 21 and 24, etc. In DVD commentaries, Doc Hammer and Jackson Publick point this trope out and confess that this is because whenever they're stuck or bored while writing they just make up two characters who are basically each other and goof around doing voices as them.
Jody, Ophelia, and Roger are often demoted to Those Three Guys in The Life and Times of Juniper Lee. Then there's the two monsters who show up from time to time who invoke this trope.