"You may also complete the ride two-up (with one person doing the piloting, the other being a passenger the entire distance - you may not share piloting duties!). In these cases, the Iron Butt Association issues one certificate, with both rider and passenger on it, but we provide two copies of it; one for the rider and one for the passenger. There is NO additional charge for a passenger, however, if they would like a pin or plate frame, you will need to add that in on the fee schedule page."
Just include a note that you want her certified as well. She will receive her own IBA member number.