Is it permissible for a woman to combine the intention of making up for missed fasts with the intention of fasting the six days of Shawwal?

This issue is known as the issue of Tashreek (combining two acts of worship with a single intention).

The ruling on Tashreek can be summarized as follows: - Tashreek is valid if the two acts of worship are means (to an end) or overlap, such as the ghusl (ritual bath) for Friday and for janabah (major ritual impurity), or if one of the two acts of worship is not intended independently while the other is, such as tahiyyat al-masjid (greeting the mosque prayer) performed with a fard (obligatory) or sunnah (supererogatory) prayer. - Tashreek is not valid between two acts of worship that are intended independently, such as the Dhuhr prayer and its ratibah (accompanying sunnah prayer), or an obligatory fast (performance or make-up) with a supererogatory fast like the six days of Shawwal, because each act of worship is independent and intended on its own.

Therefore, making up for missed Ramadan fasts and fasting the six days of Shawwal are two distinct acts of worship, and it is not permissible to combine them with a single intention. If one fasts with the intention of making up for Ramadan only during Shawwal and it coincides with the six days, it is more likely that they will be rewarded for that, but not the full reward for one who fasts them purely as a voluntary act.

Al-Sharqawi said in his commentary on Al-Tahrir by Sheikh Zakariya Al-Ansari: "And if he fasts during it - meaning Shawwal - as a make-up for Ramadan or another fast, whether a vow or another supererogatory fast, he will attain the reward of its voluntary nature, since the basis is the existence of the fast during the six days of Shawwal... However, he will not attain the full reward associated with the desired act unless he intends to fast them specifically for the six days of Shawwal, especially one who missed Ramadan, because it cannot be said that he fasted Ramadan and followed it with six days of Shawwal."