They should have separate wills, even if one was a 'mirror' of the other. Your father's will remains valid after the death of your mother, although he might want to review it now.
I've never heard of a SINGLE document being used as a will by two people and it seems extremely unlikely to me that such a document exists. (I was once the MD of a will-drafting firm, so I've studied the subject quite a lot over the years). I feel sure that your father will have his own will which, as I've said, continues to remain completely valid.