each other / one another
Reciprocal pronouns for two versus for three or more.
Each other is the usual form for two: The two clerks helped each other. One another is the usual form for three or more: The four clerks helped one another. Using each other for a larger group is common. Keep one another when the number is clearly more than two.
Examples
UseThe pair blamed each other.
UseThe three blamed one another.
AvoidThe pair blamed one another. (If you want the two-person form.)