～同士

About

同士どうし is a noun that means "fellow; mutual; comrade; companion". When used as a suffix, 同士どうし turns singular nouns into groups of two or more, and adds a nuance of solidarity or companionship.

E.g. しおりとボブは恋人こいびと同士どうしだ。 "Shiori and Bob are lovers."

The suffix 同士どうし is specifically for people.

The similar noun, 同志どうし, also describes common groups of people but is used as a standalone noun, rather than a suffix. 同志どうし is specifically used for people with similar goals/aspirations/ideals.

Summary

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  同士どうし is a suffix that attaches to nouns

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  同士どうし can only be used to describe people

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  同士どうし turns singular nouns into groups of two or more, and adds a nuance of solidarity or companionship