Praise be to Allah, the Lord of the Worlds.
Scholars have set conditions for the permissibility of joining two prayers, but they didn`t state that both must be led by the same Imam. Therefore, it is permissible, when joining Maghrib and Isha`, that each is led by a different Imam. An-Nawawi(May Allah have mercy on him) said, "According to the more correct of the two scholarly opinions, if the Imam makes final Tasleem, it is permissible for those who came late to complete their prayer behind another Imam."{al-Majmou`, 4/245}.
In conclusion, if the Imam passed out after he had led Maghrib prayer, then it is permissible for the prayer performers to choose one from among themselves to lead the Isha` prayer. This is provided that the interval between the two prayers isn’t` long, because incessancy is a condition for the validity of joining prayers. And Allah knows best.