Dought & clear, - It is not valid to combine making up missed Ramadaan fasts with fasting six days of Shawwaal with one intention

Is it permissible for me to fast the six days of Shawwaal with the
same intention as making up the days I did not fast in Ramadaan
because of menstruation?.
Praise be to Allaah.
That is not valid, becausefasting the six days of Shawwaal can only be
done after fasting Ramadaan in full.
Shaykh Ibn 'Uthaymeen said inFataawa al-Siyaam(438):
Whoever fasts the day of'Arafah, or the day of 'Ashoora', but still
owes days from Ramadaan, hisfast is valid, but if he intends to fast
this day to make up for a missed Ramadaan fast, he will have two
rewards: the reward for the day of 'Arafah or 'Ashoora' along with the
reward for making up the missed fast. This has to do with voluntary
fasts in general that are not connected to Ramadaan.With regard to
fasting the six days of Shawwaal, they are connected to Ramadaan and
can only done after making up missed Ramadaan fasts. If he fasts them
before making up missed Ramadaan fasts he will not attain that reward,
because the Prophet(peace and blessings of Allaah be upon him) said:
"Whoever fasts Ramadaan then follows it with six days of Shawwaal, it
will be as ifhe fasted for a lifetime." It is well known that whoever
still owes days from Ramadaan is not regarded as having fasted
Ramadaan until he makes up the days hemissed.