Can One Make A Dual Intention When Fasting (i.e. for making up their obligatory fasts and, at the same time, the six days of Shawwal)?
There is a saying that one can start making up their obligatory fasts now and at the same time make a dual intention for the six days of Shawwal too, on the same single day, to try to kill two birds with a stone.
We can say this is not a strong opinion, and it’s a minority opinion. Although, this is something permissible for general matters such as combining the intention for tahiyyat al-masjid and the sunnah of wudhu’ and the two sunnah before dhuhr for example, or combining the intention for the voluntary fast of Monday and the fast for the middle three bright days of the lunar month etc.
However, it is not correct to do it in this qadau and Shawwal case because you have (a) a specific obligation to make up, and (b) a specific voluntary fast to perform, which require separate intentions and focus, quite apart from the fact that the hadith concerning the six days of Shawwal makes it clear that they follow the fasts of Ramadhaan, and that would not be possible if the fast itself is both an owing Ramadhaan fast and a Shawwal fast at the same time.
Thus, these must be done separately. As I have mentioned above, it is better that one does the obligatory qadou first, and then the six days of Shawwal separately afterwards. Or, they do the six days of Shawwal now and then do the qadou afterwards. But not together on the same day with the same intention. This is the position of the vast majority of the scholars which evidences seem stronger.