In Drow Tales, after a long and bloody war between the Sharen and Sarghress clans, who have long considered one another to be their Arch-Enemy, Sabbror agrees to turn over all Sharen holdings in exchange for the Sarghress clan's help in getting revenge against the traitors that condemned the Sharen to a Fate Worse Than Death.
Schlock Mercenary Defies this with Maxim 29 from The Seventy Maxims of Maximally Effective Mercenaries: "The enemy of my enemy is my enemy's enemy. No more. No less." but the actual trope does come into play at least once. True to the maxim, both sides will invariably race to put their knife in the others' back first.
During the "Dangerous Days" arc of Sluggy Freelance, the gang's (first) trump card against Hereti Corp is Oasis, who had kidnapped or tried to kill most of them in the past. Only her devotion to Torg and mind control programming kept her somewhat on their side.
Also, due to his Sociopathic Hero nature, almost any time Bun-Bun is persuaded to help the other characters has shades of this.
In Tower of God, after a Mêlée à Trois between Viole, Arkraptor and Kang Horyang seemed to have no end, Arkraptor and Horyang decided to team up against Viole. It didn't work.
And promptly subverted when Tarquin allies with Nale.
In the prequel, the appropriately-named Start of Darkness, Right-Eye attempts (unsuccessfully) to ally with human wizard Eugene Greenhilt, despite hating humans, in order to eliminate the threat Big Bad Xykon poses to his own goblin minions.
In Snowflame, Snowflame and his Hero Antagonists have to work together to bail out Arkham Asylum after it is flooded with fear gas.
In Captain SNES, Golbez agrees to team up with Samus Aran against Eggplant Wizard and the Monkeyspank viral AI, mainly because he knows he doesn't know enough.
Ménage à 3: The trope is briefly but eloquently demonstrated by Yuki and Sonya in this strip. They can't stand each other, but Angel just made the mistake of challenging Yuki's ego, Sonya's competitiveness, and the woman they both love.
In Guilded Age, when the beast the cultists let into the world attacks, the Peacemakers and the World's Rebellion champions agree to work together to fight it. Neither side is particularly happy about it, but both agree that the threat to innocent life is too great for them to put their own conflict before stopping it.