Would "mutual acquaintance" in Mandarin be 共同朋友 or 相識朋友?
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.