expression, noun
two of a kind; birds of a feather; two people alike
Used when two people are very similar in personality, tastes, or behavior, often with a slightly negative or ironic nuance. The phrase is a set expression.
あの二人は似たもの同士で、すぐに仲良くなった。
Those two are birds of a feather and quickly became friends.
彼らは似たもの同士だから、よく喧嘩もする。
Since they are so alike, they often argue as well.
同類 is a more general term for 'same kind' and can refer to objects or groups, while 似たもの同士 specifically refers to two people who are similar.
Compound of 似た (past form of 似る 'to resemble'), もの ('person' or 'thing'), and 同士 ('fellow, mutual'). Literally 'resembling persons together'.