Is it permissible for me to fast with two intentions at the same time?

Combining two acts of worship of the same type with a single intention is known as "tashreek" (sharing or combining intentions). This is permissible when the acts of worship overlap, or when one is intended in itself and the other for a different reason. Examples include a person in a state of janabah (major ritual impurity) bathing on Friday with the intention of both ghusl from janabah and ghusl for Friday, or performing Tahiyat al-Masjid (greeting the mosque prayer) along with the regular Sunnah prayer.

However, if the acts of worship do not overlap and each is intended for its own sake, then tashreek is not permissible. Examples include fasting to make up for missed days (qada') or for a vow (mandhoor) with the six days of Shawwal with a single intention, or fasting the day of Ashura with the Sunnah of fasting on Thursdays. In such cases, a Muslim should only intend the fasting of Ashura, and he will be hoped to receive the reward for the Sunnah of fasting on Thursdays, but he will not obtain the full reward.