Dear L,
At first I misread your question and thought you were inquiring about grandparents acting as godparents. To answer your question as you state it: the code of canon law says, " Only one male or one female sponsor or one of each sex is to be employed." That’s canon 873. So only one is required. If there are two, they have to be male and female. However, one can be the official godparent (the one who signs the book) and the other can be a witness. Practically speaking, both can ACT as godparents.
Fr. Vincent Serpa, O.P.