What is the basis for applying a single punishment when the accusation is made in a single utterance against two people?

Primary text

The justification for a single punishment when the accusation is made in one word is that the falsehood of his accusation and the vindication of their honor from the slander are manifested with that single punishment, thus fulfilling the requirement, just as if the accusation were directed only at one person. If two separate accusations are made, two punishments are required because they constitute two separate acts of slander against two distinct individuals, necessitating a punishment for each, similar to accusing the second person after the first punishment has been administered. This principle applies equally when two non-mahram women or men are accused.