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. Harry Shaw. 1975.