How does a latecomer pray in a jam‘ al-taqdim (combining Maghrib and Isha) without shortening the prayers, when he does not know which prayer it is, and how does he then complete his prayer?

If a latecomer arrives while the Imam is praying, and he does not know whether it is Maghrib or 'Isha, he should enter the prayer with the intention of Maghrib, and intend to combine 'Isha with it. If it turns out to be Maghrib, then it is clear. If it turns out to be 'Isha, then he has the choice either to separate himself after the third raka'ah, or to sit until the Imam says the taslim, and then stand up to pray 'Isha in combination. It is not permissible for him to enter the prayer with the intention of 'Isha in this situation due to the condition of sequence in combining prayers by advancing them (jam' taqdim). The intention for the prayer must be specific.