Red vs. Blue, as mentioned above, in which the red and blue teams often join up to battle O'Malley, and later the Meta.
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 Red vs Blue 3, 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.
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.