co-father-in-law

noun1

1. •
   The father of one's son- or daughter-in-law; that is, the father-in-law of one's child, or, the father of one spouse in relation to the father of the other spouse; either of two or more men whose children marry each other, such as the father of a bride vis-à-vis the father of the groom.

   • 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. — 1892