同士（どうし） ‘fellow’ is a noun that may be used in the same was as a 接尾語（せつびご） (suffix). When used this way, it conveys things that are ‘mutually (A)’, ‘fellow (A)’, or ‘(A) together’.
To use 同士（どうし）, attach it directly to the end of another noun, before expressing further information about that ‘mutual’ relationship. Alternatively, (noun + 同士（どうし） + の + noun) is also common, and expresses that noun (B) is a grouped form of noun (A).

• これは弁護士（べんごし）同士（どうし）の話（はな）しなので勝手（かって）に入（はい）って来（こ）ないで下（くだ）さい。
  This is a conversation between lawyers, so please don't barge in.
• 家族（かぞく）同士（どうし）で会話（かいわ）をするときはほとんど日本語（にほんご）です。
  When we talk between family members, we mostly use Japanese.
• 彼（かれ）は友達（ともだち）同士（どうし）の約束（やくそく）を破（やぶ）った。
  He broke a promise that was made between friends.
• お互（たが）い大人（おとな）同士（どうし）なんだから、落（お）ち着（つ）いて話（はな）し合（あ）おうよ。
  We are fellow adults, so let's calm down and talk about this.

同士（どうし） is used most often to express two different types of ‘mutuality’. These are as follows:
種類（しゅるい） - Type. (A) is the same type as (B).
関係（かんけい） - Relationship. (A) shares the same relationship as (B).

• あなたたちは本当（ほんとう）に似（に）たもの同士（どうし）ね。
  You guys are really similar (in personality).
• それは兄弟（きょうだい）同士（どうし）の問題（もんだい）でしょ？私（わたし）を巻（ま）き込（こ）まないで。
  That is an issue between siblings, isn't it? Don't pull me into it.