What is the ruling regarding someone who commits adultery with a mother and her daughter, and is he considered to have had intercourse with both vaginas, given his lack of desire to marry the daughter?

Adultery is a crime and one of the greatest major sins after polytheism (shirk) and murder. Allah the Almighty says: "And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by [legal] right, and do not commit unlawful sexual intercourse. And whoever does that will meet a penalty. The punishment will be doubled for him on the Day of Resurrection, and he will abide therein humiliated – Except for those who repent, believe and do righteous deeds; for them Allah will change their evil deeds into good ones. And Allah is Oft-Forgiving, Most Merciful." And Allah the Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way."

Therefore, this person must repent to Allah for repeatedly committing this indecency with the mother and her daughter. If he wishes to marry the mother or the daughter, the preponderant opinion is that it is permissible, because that which is unlawful does not render the lawful unlawful.