As proof that you can pull these guys from other categories, No Need for Bushido has the pair start out as part of a small group of bandits, the first moment of menace for the Heroine and the chance for the Hero to save the Heroine for the start of a Rescue Romance (or whatever). Then we think they're out of the story. Nope, they show up later, and their oblivious-to-obvious-clues nature pretty quickly becomes a running gag before they end up finding the party and starting to help them out.
Bob and Elmer from Penny and Aggie. Stan and Jack also fit the trope to some degree, especially in their earlier appearances.
Sexy Losers had Touro's two nameless friends, known only as "Sarcastic Friend" and "Swearing Friend". The Swearing Friend was enough of an Ensemble Dark Horse that he started making slightly bigger appearances than just being Touro's foil.
"The Comet and the Pirates" arc of Station V3 has "Those Two Frog-Fish Guys" who are obsessed with taking over things (the Station, an incoming comet, the coming-soon ski resort ...)
Equius and Nepeta from Homestuck are not really all that important, but they're almost never seen without at least mentioning the other, if not speaking to each other and adventurin' together. They're also Platonic Life-Partners and have engaged in many scenes to balance the mood when the story gets pretty dark.
Larry And Darryl/Daryl/Darrel from The Whiteboard. They're brothers (and appear to be twins), and originally showed up as featureless, no-necked humans. When the strip switched entirely over to furries, they became squirrels. Unless one of them is directly addressed, it's hard to tell who is which—even in universe (Word of God states that their parents sometimes have trouble telling them apart).
A lot of minor characters in Camp Weedonwantcha get paired up this way, most notably Steven and Steve. Lampshaded in one strip, where Malachi encouraged them to tell their friends about the lantern at the library, but quickly realized they would most likely have to tell strangers instead.
The Bird Feeder has Jim and Terry, two mockingbirds. They always appear together, have generally the same temperament, and generally no other characters appear in strips they're in. In their first appearances, they looked exactly alike.