He is confused about how to make up two rak’ahs when he has missed both tashahhuds
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What you have seen some of your fellow worshippers doing, making up two missed rak’ahs with two tashahhuds is not something strange in some cases. The reason for that is that when the imam says the salaam, the latecomer (the person who has missed part of the prayer with the imam) should stand up and complete his prayer. So what he prayed with the imam is the first part of his prayer, on the basis of which he should complete the rest of his prayer, because the Prophet (peace and blessings of Allaah be upon him) said: “When you come to the prayer, you should be dignified (and not rush). Whatever you catch up with, pray, and whatever you miss, complete it.” Narrated by al-Bukhaari, 635; Muslim, 603.
If a man misses the first rak’ah of Maghrib with the imam, and joins him in the second rak’ah, the second rak’ah for the imam is the first rak’ah for this latecomer. When the imam has completed the prayer, this latecomer should stand up and do the remaining rak’ah of his prayer with al-Faatihah only, and sit to recite the Tashahhud and say the salaam.
If a person joins the imam in the third rak’ah of Maghrib, it is the first rak’ah for him, so when the imam says the salaam, he should stand up and do what is left of his prayer, so he should do one rak’ah with al-Faatihah and another soorah, which will be the second rak’ah of his prayer, after which he should sit to recite the first tashahhud, then stand up to do the third rak’ah with al-Faatihah only, then sit to recite the last tashahhud and the salaam… and so on.
See also question no. 49037
And Allaah knows best.