co-wife vs fiancé

co-wife

noun
  • In a polygamous marriage, another wife of a woman’s husband. 

fiancé

noun
  • A person engaged to be married. 

  • A man who is engaged to be married; the man to whom one is engaged. 

How often have the words co-wife and fiancé occurred in a corpus of books? (source: Google Ngram Viewer )