All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, you should know that combining two prayers is only allowed in situations that the Shariah (Islamic law) permits.
Moreover, when combining Dhuhr and ‘Asr prayers, the person who offers prayer should pray Dhuhr first and conclude it by saying the Taslim, then pray ‘Asr separately and conclude it by saying the Taslim as well. The same applies when combining Maghrib and ‘Isha, Maghrib is prayed first and concluded with the Taslim, followed by ‘Isha, which is also prayed and concluded with the Taslim.
The two prayers are not to be combined with one Taslim. Rather, each is performed independently, just as if it were being prayed in its own designated time.
Furthermore, when prayers are combined, the time of both becomes one. Thus, if ‘Asr is combined with Dhuhr in advance (Jam’a Taqdim), then the entire time of Dhuhr becomes the time for both prayers. Also, if Dhuhr is delayed and combined with ‘Asr (Jam’a Ta’khir), then the entire time of ‘Asr becomes the time for both prayers.
Similarly, the same rule applies to Maghrib and ‘Isha, their time also becomes one when they are combined.
Allah knows best.