Is it permissible for more than one person to perform the Istikhara prayer for a single matter, such as a father performing Istikhara for his son concerning the same matter?

If the matter is shared between you and your father (such as a business), then it is permissible for both of you to perform Istikhara. However, if the matter is exclusive to your father, then Istikhara is permissible for him. There is a difference of opinion regarding the permissibility of you performing Istikhara on his behalf; it is preferable that he limit himself to performing Istikhara for himself.