Definition from Wiktionary, the free dictionary
co-father-in-law (plural co-fathers-in-law)
- the father of one's son- or daughter-in-law; that is, the father-in-law of one's son or daughter, or, the father of one spouse in relation to the parents of the other spouse.
- (in the plural) The relationship between people whose children marry each other; the father of the bride vis-à-vis the father of the groom.
- 1884, G. A. Grierson, “The Song of Bijai Mal”, in Journal of the Asiatic Society of Bengal, volume 53, page 132–133:
- ‘Hear, O well-read Paṇḍit, my daughter is ready for marriage. Take with you a barber's lad, go forth into the country, and search for a bridegroom. […] Search for a house worthy of my house, and search for a boy worthy of my child. Search for a co-father-in-law worthy of a co-father-in-law.’ […] Up rose the well-read Paṇḍit and said ‘O king, hear my petition. There is a co-father-in-law worthy of you as co-father-in-law. There is a boy worthy of your child. There is a house worthy of your house […] ’
- 1892, Theodōros Kolokotrōnēs; (trans.) Elizabeth Mayhew Waller Edmonds, Kolokotrones the klepht and the warrior: Sixty years of peril and daring. An autobiography, page 104:
- my old friend […] whom I trusted greatly, because I had taken care of his family in former times, and because he was, as it were, my co-father-in-law, for I had betrothed my daughter to his son.
- 1996, John W. Freeman, The Metropolitan Opera stories of the great operas, page 208:
- Diègue tries to treat Gormas amicably as his prospective co-father-in-law, but Gormas provokes him with rude remarks and slaps him
Usage notes 
- (uncommon) Found primarily in translation. In conversation, the generic "in-law" is generally used, with context left to disambiguate. Once grandchildren are born, the term co-grandfather may be used if the focus is on the relationship through the grandchildren rather than though the married couple.
Coordinate terms 
Related terms 
father of one's son- or daughter-in-law