Is the statement 'O son of two adulterers' considered one or two acts of slander (Qadhf)?

Primary text

The statement made to a man, 'O son of two adulterers' (Ya ibna az-zaniyyayn), constitutes a single act of slander against both of his parents. If both parents are deceased, the right to punishment is established for their son, and only one instance of the prescribed Hadd penalty is required, according to a single established view. The supporting evidence is derived from the principle of consolidating the offense into one statement concerning both parents' alleged acts.