Rivals rarely teaming up

[Warmmedown]

It seems that either you know things are serious when they team up or you see that they would be a formidable team.

Sanji and Zoro in One Piece. They teamed up a few times. It was always during a very tough battle or an existential threat for the crew.

Hijikata and Okita in Gintama. They work together a lot as police, but I only remember them teaming up to fight once.

 

[Yoshi]

Naruto and Sasuke rarely team up.