Haloid. When Samus and MC first meet, Samus attacks MC for no good reason and they start a running battle. When more Covenant troops show up, Samus and MC join forces to fight them, including a Back-to-Back Badasses moment.
In the League of Legends cinematic "A Twist of Fate" Garen and Annie fight against Katarina and Tryndamere, until the Baron Nashor shows up. Naturally, they choose to team up against the greater menace.
Red vs. Blue: Happens quite a lot as the series goes on. The Red and Blue teams first join forces to battle O'Malley, then later the Meta, and later still, threats like the remains of Project Freelancer or Felix. While they still don't like each other very much, they've long since ceased to be true enemies, and now work together more often than not.
In Red vs Blue 3, (no relation to the Machinima series by Rooster Teeth) Red and Blue are about to have their epic showdown when Green comes in and beats them up. The result? Red and Blue perform a Fusion Dance to create Purple, who swiftly kills Green in the span of less than 10 seconds.