Each other and one another are both written as word sequences, but it is better to treat them as compound pronouns rather than as combinations of two pronouns. At the same time, they correspond the correlative use of each . . . other and one . . . another (cf 6.58) in sentences such as:
They each blamed the other.
The passengers disembarked one after another
先の記事 ([2020-09-04-1]) で解説したように，歴史的には上の例文にあるような構成要素が離ればなれの相関構文が先にあり，後になってその特殊な形として構成要素が隣接した each other, one another が生じたということだった．
Each other and one another resemble reflexive pronouns in that they cannot be used naturally in subject position. Instead of , [1a] is preferred.
?The twins wanted to know what each other were/was doing. 
Each of the twins wanted to know what the other was doing. [1a]
There appears to be no such constraint on reciprocals as subject in nonfinite verb clauses:
The twins wanted each other to be present at all times.
However, the rule which excludes occurrence in subject position holds not only for independent pronominal use but also for genitival reciprocals in subject noun phrases. The reciprocals must have coreference with antecedent phrases which have some other genitive or possessive modification. Compare:
*Each other's letters ----------
?The letters to each other ---------- were delivered by a servant.
Their letters to each other ----------
・ Quirk, Randolph, Sidney Greenbaum, Geoffrey Leech, and Jan Svartvik. A Comprehensive Grammar of the English Language. London: Longman, 1985.
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