‘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).
This is a conversation between lawyers, so please don't barge in.
When we talk between family members, we mostly use Japanese.
He broke a promise that was made between friends.
We are fellow adults, so let's calm down and talk about this.
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).
You guys are really similar (in personality).
That is an issue between siblings, isn't it? Don't pull me into it.
is most frequently used to highlight ‘people’, or ‘living things’ that share some type of relationship, rather than inanimate objects. An easy way to remember this is that the 士（し） kanji in this word means ‘gentleman’. However, it may occasionally be seen in reference to inanimate objects as well.
This glue sticks really well between paper, but between plastic, it does not stick well. (Natural, but not very common)
Watching a fight between two professional martial artists is fun. (Natural)