Praise be to Allah.
The answer to that is that we would say to the imam first of all: You should not remain silent for this long time between reciting al-Faatihah and reciting what comes after it. What is prescribed is for the Imam to remain quiet briefly between al-Faatihah and the soorah that comes after it so as to distinguish between the obligatory recitation and the mustahabb recitation. During this silent pause it is prescribed for the one who is praying behind the Imam to recite al-Faatihah and complete the recitation, even if the imam recited it. But as for the imam falling silent for a long time, this is contrary to the Sunnah. Moreover, even if we assume that the imam did that and remained silent for a long time, if the person praying behind him recited al-Faatihah and completed it, he may recite a soorah after that until the imam begins to recite the soorah that comes after al-Faatihah, at which point he should stop reciting, because it is not permissible for the person praying behind the imam to recite when the imam is reciting, except with regard to reciting al-Faatihah only. End quote.