Combining making up a missed Ramadan fast with the fast of ‘Ashoora’ or ‘Arafah

Praise
be to Allah.

This issue is known to the scholars as the
issue of pairing or combining acts of worship. It takes many forms such as
this one, which is combining an obligatory and a mustahabb action with one
intention. If a person intends to do the mustahabb action, that will not be
acceptable for the obligatory action. If a person fasts with the intention
of ‘Ashoora’, that will not be acceptable as making up for a missed Ramadan
fast, but if he intends to make up the missed Ramadan fast and does that on
the day of ‘Ashoora’, then his making up the missed fast will be valid, and
there is the hope that he will also attain the reward for ‘Ashoora’,
according to some scholars. 

Ar-Ramli (may Allah have mercy on him) said
in Nihaayat al-Muhtaaj (3/208): If he fasts in Shawwaal to make up a
missed fast, or in fulfilment of a vow, or otherwise, or on an occasion such
as the day of ‘Ashoora’, he will also attain the reward for the voluntary
fast. This was stated in a fatwa by my father (may Allah have mercy on him),
following al-Baarizi, al-Asfooni, an-Naashiri, the faqeeh ‘Ali ibn Saalih
al-Hadrami, and others, but he will not attain the full reward that comes
from doing what is required, especially if he missed Ramadan and fasted
Shawwaal to make up for it.

End quote. 

It says something similar in Mughni
al-Muhtaaj (2/184) and Hawaashi Tuhfat al-Muhtaaj (3/457) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy
on him) said in Fataawa as-Siyaam (438): If a person fasts the day of
‘Arafah or the day of ‘Ashoora’, and he still owes fasts from Ramadan, his
fast will be valid, but if he intended to fast that day to make up for a
missed Ramadan fast, he will have two rewards: the reward of the day of
‘Arafah or the day of ‘Ashoora’, and the reward for making up the missed
fast. This applies to voluntary fasts in general, that have nothing to do
with Ramadan. But with regard to the six days of Shawwaal, they are
connected to Ramadan, and can only be done after making it up. If a person
fasts these days before making up missed Ramadan fasts, he will not attain
the reward for them, because the Prophet (blessings and peace of Allah be
upon him) said: “Whoever fasts Ramadan then follows it with six days of
Shawwaal, it will be as if he fasted for a lifetime.” And it is well known
that if a person owes missed fasts, then he cannot be regarded as having
fasted Ramadan until he completes the days that he owes.

End quote. 

So what one should do is hasten to make up
what one owes; this is more important than doing voluntary actions. But if
he does not have much time and he cannot make up all the days that he owes,
and he is afraid of missing out on the fast of a day of virtue such as
‘Ashoora’ or the day of ‘Arafah, then he may fast with the intention of
making up the missed fast, and there is the hope that he will attain the
reward of ‘Ashoora’ and ‘Arafah as well, for the bounty of Allah is
immense. 

And Allah knows best.