Hazel and Cha-Cha of The Umbrella Academy. They like the simple things in life. Candy, cookies, pie. And dismembering innocent people with hack saws while plotting nuclear Armageddon.
Burt Schlubb (Fat Man) and Douglas Klump (Little Boy) from Sin City aren't exactly killers, but still somewhat fit.
Fite and Maad, agents of APES, from The DCU's Young Justice. Ironically, they're much more pragmatic than some of the series' other antagonists... but their goals are often much more cruel. By the end, possibly due to intentional Villain Decay, they're the Overprotective Dad of YJ's newest member, and his wacky friend.
The Mauler Twins are like this (with the rapport and the squabbling and the being evil thing), but they're Mad Scientists, not assassins or anything like that.
Magmaniac and Tether Tyrant are a more straight example.
Spider-Man's foes Styx and Stone (they'll break your bones!) and several other Marvel Comics villains, like Knight and Fogg, Hammer and Anvil, Brother Sun and Sister Moon, and the Brothers Grimm.
While in Madripoor, Wolverine frequently ran afoul of two legbreakers named Roughhouse and Bloodscream. Originally believed to be mutants, Roughhouse was later revealed to be an Asgardian troll, while Bloodscream is a five-hundred-year old man who gained very long life and vampire-like abilities when treated by a necromancer he thought was a doctor. How Roughhouse and Bloodscream became partners and criminal enforcers is anybody's guess. The gangsters Reno and Molokai, who appear in several issues of Wolverine and murdered Jubilee's parents, also seem to fit this trope.
The Satan Brothers in DC'sLobo; again a Blues Brothers parody.
Nightwing villains and Evil Albino twins the Pierce brothers, who, despite looking and dressing exactly the same, do have rather distinct personalities; Barry is a megalomaniac who tends to think only in terms of himself, while Buddy is somewhat dimwitted and easily manipulated.
Rob and Don in The Dark Knight Returns blur the line between this and Those Two Guys; They're dangerous gang members, and fanatically attach themselves to the most powerful group, but they're comparatively harmless and mostly comment on other peoples' actions. Batman beats them up at least twice, but they end up undergoing a Heel–Face Turn when Batman deputizes them and the rest of the Sons of the Batman gang.
Cannon and Saber from the Vigilante comic in The DCU. Word Of God confirms that this pair are gay. Cannon later appeared in Final Crisis Aftermath: Ink with a new partner, Slipknot, though it's not clear if this is the same Cannon.
The Mighty Thor adversaries Mr. Hyde and King Cobra. (Well, until they had a falling out and Cobra told Hyde he was sick of him.)
X-Men villains the Juggernaut and Black Tom Cassidy almost exclusively worked together after a while. Similarly, the Blob and Unus the Untouchable were inseparable and worked together even when not in a larger team.
Daredevil ran up against a pair of thugs named Turk and Grotto for years during and after Frank Miller's run. More recently, Ed Brubaker introduced two street level criminals named Chico and Merv who are based on Brian Posehn and Patton Oswalt.
Idget the Midget and Dangerous Dan McBoo in Mickey Mouse comics. Idget is slightly smarter than Dan, but they in general seem equal partners in crime — sometimes employed by others, sometimes working on their own.
Doom Patrol villains Monsieur Mallah & brain — who take this to the extreme — one's a brain in a can, the other's a gorilla... they also really love discussing philosophy — and each other.
Arguably, Trypticon and his yes-man Wipe-out in the Transformers comics; They're both persisting menaces that are only barely affiliated with one side; Wipe-Out's primary role is just doing things for the not-exactly-mobile transforming city, and they aren't exactly equals or anything, but....
The Battlechargers are an old example: Runabout and Runamuck, who mostly commit petty vandalism while giggling at everything. Many fans think they're the precursors to Beavis And Butthead.
The Harley and Ivy miniseries featured Slash and Burn, who were all but directly stated to be, uh, close.
School bullies Derreck and Onion from Curtis, although while they frighten the protagonist and his brother, they're rather incompetent as bad guys otherwise.