Answer: It is narrated by Abu Ayub Al-Ansari the Prophet (peace and blessings be upon him) said: "Whoever fasts Ramadan and follows it with six days of Shawwal is as though he has fasted the entire year." Muslim

It is also reported by Thawban that the Messenger of Allah (Allah bless him & give him peace) said: “Fasting Ramadhan is like fasting ten months, and fasting the six days is like fasting two months, so that amounts to fasting the entire year.” ( Musnad Ahmed)

The Qadha of Ramadhan and the fast of the  six days of Shawwal are two specific and distinct actions and hence they cannot be combined with one intention.  Each has to be observed separately. 

The intention of the fast must be specific and not dual. One cannot make two intentions for one day’s fast. The person should make an intention for Qadhaa only or Nafl only.

And Almighty Allah Knows Best

Yusuf Moosagie