Is expiation (kaffarah) obligatory upon a couple if the husband has intercourse with his wife during the daytime in Ramadan while traveling and they are not fasting, and they were unaware of the impermissibility of this, and specifically upon which of them is expiation (kaffarah) obligatory?

Allah has permitted travelers to break their fast in Ramadan as a facilitation and to remove hardship, and they are obligated to make up the days they missed. Therefore, as long as you and your husband broke your fast during your journey, there is no sin upon you regarding other things that break the fast, such as eating, drinking, and marital intimacy, and no expiation is required of you, because you are in a state of not fasting, and not fasting is indivisible.