Question: I have heard that it is written in one of the commentary on Birghiwi's Wasiyyatnama, "A person who has fard salats to make up should perform sunnat salats with the intention of both performing sunnat salats and making up his/her qada salats." Is it true?
ANSWER
Yes, it is true. In the original text that we have provided as a link below, it is stated that it is necessary for one who has many qada salats to perform sunnat salats with the intention of both performing sunnat salats and making up one's qada salats. If one performs the sunnat salat of Salat al-Maghrib as three rak'ats [with the intention of making up a missed Salat al-Maghrib] and performs the post-fard sunnat salat of Salat al-Isha as three rak'ats [with the intention of making up a missed Salat al-Witr] and recites Qunut prayers [in the third rak'at], it is written therein that one will have performed both sunnat salats and qada salats. The reason for this is that what matters is not the number of rak'ats but performing an extra salat in addition to a fard salat. Performing three rak'ats, too, fulfills the sunnat. In order to emphasize the validity of double intention, it is also stated therein, "Two duties can be lifted by a single act, just as sajda at-tilawat can be done with a ruku' in salat." (p. 263)

Click here for the original text