What is it called when an existence in two universes takes a separate course of action in each?

Not really sure if this is Physics or Philosophy... Anyway in accordance with the multiverse theory at a given point there's 2 versions of a person, each one in an identical universe. When in one universe the person does something and in the other the person does something different, and some sort of divergence occurs. I think this splitting of fate has a scientific name, but I can't remember what it is... I'm probably just insane but I think I've read about this before...