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)