In a bris where both grandfathers are still alive and it’s the first for both, who typically acts as the sandek? Is it customary for the father’s dad or the mother’s dad? Would like to avoid any controversy.
Because of the obligation of kibbud av, it is proper to give the first honor of sandaka’us to the paternal grandfather. However, this is not a hard set rule, and some have the custom of giving the father in law first.
Sources: Derech Sichah (questions and answers from Rav Chaim Kanievsky) writes that the grandfather (father’s father) should be given the first right to the sandaka’us, though he doesn’t directly address the question of the father-in-law. See Chacham Tzvi, no. 70, who states clearly that a father comes before a father-in-law (see also Rabbi Akiva Eiger, Yoreh De’ah 251).