Praise be to Allaah.
With regard to the adhkaar, it seems that it is sufficient to
say them only once, because it is as if the two prayers have become one, so
it is sufficient to say the adhkaar once. But one should stick to that which
is more general. For example, when combining Maghrib and ‘Isha’, in the case
of Maghrib it is Sunnah to mention Allaah ten times [i.e., to say Laa
ilaaha ill-Allaah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa
huwa ‘ala kulli shay’in qadeer (There is no god but Allaah alone with no
partner or associate; His is the power, to Him be praise, and He is able to
do all things) ten times], and in the case of ‘Isha’ it is Sunnah to say it
three times, so one should choose the higher number, because the lower
number is included in the higher number. But if he recites the dhikr for
each, I do not think there is anything wrong with that, but the former
option is sufficient. End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him)
Liqaa’aat al-Baab il-Maftooh (1/259).