Which would be the more authentic Mandarin translation for "mutual acquaintance":
1) 共同朋友 2) 相識朋友?
Is it the same for Cantonese?
Chinese denotes the concept("mutual acquaintance") differently. These are the idiomatic sentences in mandarin:
我们两个都认识他 // both of us have an acquaintance with him. Or he is a mutual acquaintance of ours.
张三和李四都和我相识 // both 张三 and 李四 have an acquaintance with me.
张三和李四都认识王五 // both 张三 and 李四 have an acquaintance with 王五.
However, when it comes to the term "mutual friend", you can put 共同的朋友. For example,
张三是我们共同的朋友 // 张三 is a mutual friend of ours.
Chinese don't usually use the term "mutual acquaintance" (共同的相识)
The more common way to describe a mutual acquaintance would be like the example below:
Tony is a [mutual acquaintance] of you and I
托尼是你跟我[都認識的熟人] (Mandarin) Tony is an acquaintance of both you and I
東尼係你同我[都識得嘅熟人] (Cantonese) Tony is an acquaintance of both you and I
According to how mutual acquaintance is often used, it should mean 共同的相识。
Acquaintance is not yet a friend, a friend is 朋友，an acquaintance is 相识的人。
I am not sure how you speak in Cantonese, but they write the same.