What is the ruling if a man engages in sexual intercourse with his wife during the fasting hours of an obligatory make up fast for either of them?

Engaging in sexual intercourse during the fasting hours of Ramadan
If a man is fasting and engages in sexual intercourse during the fasting hours of Ramadan, he must:
• Make the maximum kaffara [expiation].
• Make up the fast.
• Fast sixty consecutive days.

In addition, he is to repent from this sin by regretting having committed it and resolving never to commit it again.

If it was only the wife who was fasting, he is not obliged to make kaffara or make up the fast. As for the woman who breaks her fast by engaging in sexual intercourse during an obligatory fast, she must only make up the fast and is not obliged to make kaffara because the Prophet (peace and blessings be upon him) ordered the man who violated his fast in Ramadan to make kaffara and did not order him to tell his wife to do likewise. Since the principle is that it is not permissible to delay the clarification of a matter beyond its time, it has become known that a woman in such circumstances is only to make up the fast and repent. But if she was not fasting, she is not obliged to either make up the fast or make kaffara.
Engaging in sexual intercourse during the fasting hours of a make up fast of Ramadan

Al-Qurtubi mentioned in his Tafsir that the majority of scholars have maintained that a person who violates a make-up fast of Ramadan by engaging in sexual intercourse is blameworthy. A person who violates his fast-day in this manner must therefore make up this day, but is not obliged to make kaffara. They based their opinion on the hadith narrated by Um Hani (may God be pleased with her) who said that on the days of Mecca's conquest, the Prophet (peace and blessings be upon him) offered her to drink from his cup after he had drunk first. After drinking, she said: "O Messenger of God! I was fasting but I did not wish to decline the generosity of God’s Messenger." He asked her, "Were you making up a fast?” “No," she replied. He then said, "Then there is no harm" (recorded by Abu Dawud, Al-Baihaqi and others).
In Al-Nisa`i's version of the hadith recorded in Al-Sunan Al-Kubra, the Prophet (peace and blessings be upon him) told Um Hani: "If you are making up a missed day of Ramadan, then fast a day in its place. If not, you may do as you please — either make up a day in its place or not." Due to this, Ibn Rushd wrote in Bidayat Al-Mujtahid (vol. 1, p. 224): "The majority of scholars are unanimous that kaffara is not due upon a person who deliberately breaks a make up fast of Ramadan because it does not have the same sanctity i.e. of Ramadan."
The ruling
The inquirer must fast another day in lieu of the make-up fast-day of Ramadan and, according to the opinion of the majority of scholars, is not obliged to make kaffara.

God the Almighty knows best.