For example: someone prays Maghrib behind an imam who is praying Isha. When the imam rises for the fourth rak'ah of Isha, the follower has two options:

Either he separates from the imam, and this presents a problem;

or he follows the imam, and this too presents a problem, because if he follows the imam, he adds an extra rak'ah, whereas if he remains behind, he differs from the imam. Yet the Prophet ﷺ said, “The imam has only been appointed to be followed.” (1) Does this situation fall under the sounder, preponderant view that a difference in intention between the two prayers does not matter?

The answer is: yes, it falls under the preponderant view, and it is permissible for someone to pray Maghrib behind one who is praying Isha. This occurs frequently. If he joins the imam in the second rak'ah or later, there is no problem, for he follows his imam and gives the salām with him. If he joins in the third rak'ah, he performs one rak'ah after the imam; and if he joins in the fourth, he performs two rak'ahs. But if he joins in the first rak'ah, then when the imam rises for the fourth, he must remain seated and not rise.

However, when he remains seated, should he intend to separate from the imam and give the salām, or should he wait for the imam?

The answer is that he may choose either option. However, we recommend that he intend to separate and give the salām, if he can still catch what remains of the Isha prayer with the imam, so that he may attain the congregational Isha prayer.

If someone asks: Why do you permit him to separate when the imam must be followed?

The answer is: because of a legally recognized excuse. Separating from the imam due to a legally recognized or physical excuse is permissible.