mutual crush; two people who like each other; lovers
Refers to two people who have romantic feelings for each other, often used in the context of a budding relationship or when emphasizing that the feeling is mutual. Can be used as a noun to describe the pair or their relationship.
Those two like each other, but they just won't start going out.
If you both like each other, you should just be honest about your feelings.
両思い also means mutual love, but is more abstract and can describe the state of mutual affection, whereas 好き同士 specifically refers to the two people involved.
Compound of 好き (like, love) and 同士 (mutual, fellow). The combination literally means 'mutual liking' and refers to two people who like each other.