Harold Ramis shot an extra scene for Caddyshack in which two former SNLer improvise a together even though they actually hated one another And the only movie did Who?

You're thinking of the scene in Caddyshack where Bill Murray and Chevy Chase improvise a scene together, despite their real-life animosity.

The scene involves Murray's character, Carl Spackler, walking past Chase's character, Ty Webb, and spontaneously launching into a bizarre, improvised rant about gophers and the rules of golf.

However, there isn't a confirmed story about them actually hating each other. While they had a bit of a rivalry during their time at SNL, they have spoken about getting along fine on the Caddyshack set.

It's important to remember that anecdotes and rumors about Hollywood personalities can sometimes be exaggerated.