Is it permissible to combine the intention to fast the missed days of Ramadan with the six days from Shawwal?

It is preferable to make up the missed fast days of Ramadan before fasting the six days from Shawwal if possible. This is due to the hadith that states, "A debt owed to God has more right to be fulfilled."

According to Shafi'i scholars, it is possible to combine the intention of making up the missed fast days of Ramadan with the six days from Shawwal. Alternatively, one can fast the six days from Shawwal first and delay making up the missed fast days of Ramadan provided one completes them before the next Ramadan.

The time for making the intention
The intention to fast the six days from Shawwal can be made any time before noon provided one has not done anything that vitiates the fast. This is the case for all voluntary fasts, in contrast to obligatory and make-up fasts for which one must make the intention before dawn.

And God Almighty knows best.