***NOT A TEACHER***mnstar, good morning. I think that the "rules" say that you should use "each other" for two persons: Tom and Mona have known each other for five years. You should use "one another" for three or more persons: All the students in Mr. Smith's class are helping one another with the lesson. Here in the United States, many people do NOT follow this rule. They use "each other" in almost all sentences. Thank you.
It's a rule that is also widely ignored in the UK, where they are often used interchangeably. In all honesty, I think that in British English, the rule can be regarded as dead and buried.
