Akademik

each other, one another
In standard speech, each other is used when two persons are involved; one another is preferred when three or more persons are concerned: "The man and his wife spoke to each other excitedly." "The six motorcyclists were arguing with one another." Common usage (not recommended, however) permits such a statement as "The five culprits regarded each other with distrust." Be safe: use one another when three or more persons are involved.

Dictionary of problem words and expressions. . 1975.