'Allo 'Allo!: Cairstairs and Fairfax are those two British airmen in the same uniform, both are brunets and both sport a moustache. They always wear the same generic outfit as a part of their escape plan. They share a Catch Phrase: "Hello!" "Hello!" They are practically undistinguishable.
Cordelia usually ends up as Giles's research partner while Buffy, Xander, Willow, and Oz are out on patrol. Hilarity Ensues.
Jonathan and Andrew.
Forrest with Graham.
In Mash, there are Frank and Margaret in the first few seasons, whose status is lampshaded on several occasions. They are easy to recognize once you know which major of the duo is a lady.
Officer: Which one is Burns and which is Houlihan?
Hawkeye: It doesn't matter. They're interchangeable.
Bulk and Skull, the school bullies in Mighty Morphin' Power Rangers. Unlike most sets of Those Two Guys, they remained constant characters in every season of the continuous plotline, cameoed in the first season after the continuous plotline, and even cameoed in the 10th anniversary Reunion Show.
Spiritual successors of Bulk and Skull from Mighty Morphin Power Rangers are Cassidy and Devin in Power Rangers Dino Thunder, an aspiring reporter and her cameraman.
Bulk himself returns in Power Rangers Samurai in this role for the first time in more than a decade, though Skull's son takes his father's place in the onscreen duo. This role gives Bulk the most appearances of any character in the series with nine seasons (As a regular in the six Zordon Era seasons, with a couple appearances in Power Rangers Lost Galaxy, a cameo in Power Rangers Wild Force, and as a regular again in Samurai), having formerly been tied with his friend Skull and Tommy Oliver. Though Skull is set to appear in the finale of Samurai, which would tie him with Bulk.
The West Wing's Ed and Larry. Or Larry and Ed. They're in every meeting, advise the President on every issue and no one knows who the hell they are. Lampshaded a couple of times when they are given the wrong folders, and when someone asks if they always walk around together, or the time when the pair are introduced to a new character and when they ask "which one is Ed and which is Larry?", Ed, Larry and Josh all respond "it doesn't matter".
They had Frankie and Pete for seven years. Frankie and Pete found out everything in Series 7 Episode 20 but then they had their memories erased. We were then subjected to the terrible Series 8.
There was a kid called Mikey for the first three episodes,and occasionally a kid called Dave.
Ray and Chris in Life On Mars, with Ray particularly embodying the values we've thankfully left behind.
Subverted in Ashes to Ashes wherein they become main characters and when in the finale it is revealed that Ray and Chris are also from the real world.
Primeval has Tom and Duncan; Conner's geeky college friends who aren't in on the secret. Tom later contracts The Virus from a dodo and dies, and when Duncan finally reappears again in season 4, he has become a paranoid and reclusive Conspiracy Theorist.
Lost had Steve and Scott until one of them died (no-one can remember which). They very rarely appeared and rarely had few lines.
Charlie and Hurley in the first two seasons or so, whose stories were largely separate from the core mysteries of the show and would sometimes be shown asking, "What's going on?" to the main characters and commenting sarcastically on, "There goes Jack and Kate off into the jungle again, up to who knows what?"
A retconned example would be Nikki and Paolo, who were apparently there all along observing events until they got involved in the main story and promptly died in the third season (due to fan dissatisfaction with these characters showing up from nowhere and them being expected to give a damn).
It has an extended version with Blair's entourage: an Asian-American nerd, a (possibly Hispanic for some extent) Queen Bee wannabe, an African-American beautiful girl an... well, the other one. For season three, it's done with Jenny's courtship.
Isabel and Kati, particularly in the first season.
Matt Rutherford (the black guy) and Mike Chang (the Asian guy) from Glee. Only in the first season, though.
Subverted in 30 Rock, with Grizz and Dotcom being anything but ordinary. They are, however, as close as "those two guys" can be for a man like Tracy.
The writer Robert Holmes was fond of having two minor characters in his serials whose main purpose was hanging around in the background being funny and commenting on the story, to the point that Whovians refer to Those Two Guys as the "Holmesian Double Act".
Except the Holmesian Double Act is very rarely minor characters. The iconic "double act" are Jago and Lightfoot from Talons of Weng-Chiang. Jago owns the theatre where Weng-Chiang is hidden and his agent plies his trade, and Lightfoot is a police pathologist and the Doctor's de facto landlord for the story. Both important to the plot to the point that they are more important then the Doctor until the finale.
In fact; Jago and Litefoot were so popular they eventually got their own series
The trope gets parodied mercilessly in the revived series episode A Good Man Goes To War, with the introduction of the "thin one" and the fat one"
Jeffrey and Lester. These two may also be considered a Beta Couple to Heterosexual Life-Partners Chuck and Morgan. They have a rock duo called Jeffster, which is (unsurprisingly) terrible, or terribly awesome. Also, while they can be Those Two Guys in some episodes, they have some more in-depth treatment. First of all, they aren't indistinguishable from each other. Second, they aren't clueless about the show's Masquerade: it's revealed near the end of Season 2 that they know more about the Power Trio than anyone else, although they have still guessed wrong in a big way.
One-off characters Matty and Scotty have many elements of Those Two Guys. They're two stereotypical mobsters who have almost identical personalities. One is somewhat heavier in build than the other, but otherwise unless you're paying attention it's hard to tell which is which. They are more central to the plot than is typical for this trope, but don't actually do all that much themselves and spend most of their on-camera time tagging along injecting Plucky Comic Relief, and Leaning on the Fourth Wall about Chuck and Sarah's relationship. Many viewers and critics expressed disappointment when they were unceremoniously killed off at the end by the episode's Big Bad.
In the Season 2 finale, Vir and Lennier are shown discussing their respective bosses' antics, secret agendas, clandestine meetings and unexpected character changes and commiserating over them and it is implied it is a weekly ritual.
We get two maintenance workers in a Season 5 episodes with no clue what is going on as the station comes under attack by an unknown alien race.
There's Franklin and Marcus who are pretty important to the story, but still fulfilling a secondary story function to the main characters and sometimes comment on the events of the main story.
Marcus and Ivanova occasionally veer into this territory, although their characters are very distinct, and Marcus's feelings for Ivanova complicate things.
In the new Battlestar Galactica, Dualla and Gaeta are occasionally shown talking about what is going on, but by the last two seasons that role has been taken up by Racetrack and Skulls, who frequently fly vital and extremely hazardous Raptor scouting missions without any idea what the overall mission objective is, and complain about it.
An interesting take on the idea is in HBO's Rome, where Lucius Vorenus and Titus Pullo may be two of the main characters, but they are also playing a Rosencrantz-and-Guildenstern role where their stories, although important to them, are but a minor side-show compared to the unfolding civil war between Caesar and Pompey, which they observe and sometimes affect. This angle is lost in the second season, when their stories are largely separated from the bigger, more important characters, at least until the finale when they side with their patrons (Mark Antony and Octavian, respectively) once again.
Ryan and Esposito from Castle fit very well, along with being possible darkhorses.
Brian and Kurt from the British TV series Teachers fit this trope almost exactly. Brian is the tall jock P.E teacher and Kurt is the short IT teacher. They could also be labeled as Heterosexual Life-Partners.
Whichever two lab rats happen to feature in any particular episode of CSI: Crime Scene Investigation (particularly Archie or Mandy)... their personalities are just, y'know, those two lab techs. Yet they've almost all been around, just conversing in the background for the majority of the show. Every dozen or so episodes they get thrown a bone and have an actual episode featuring them. So much so that Hodges and Wendy have been upgraded to main cast members.
Korean dramas like to feature those three guys, usually with names and a generic personality frameset: the gossipy one, the play-by-the-rules one, and the Butt Monkey.
It always seems to have a pair that last a few years, which usually means they'll be nerdy and/or wannabe players.
JT and Toby were it for Seasons 1-6 until JT was killed, then Danny and Derek for Seasons 6-7 until Danny outgrew Derek and matured, and now recently Connor and Wesley have settled into Degrassi's Those Two Guys mantle, which had people shuffling in and out of that spot since Season 8.
Horatio Hornblower has sailors Styles and Matthews, who at least by the second series start to be Those Two Guys. Both have been under Horatio's command since the first episode and are the most prominent lower-deck characters.