Must the mosque's second floor overlook the first, and not be entirely separate from it?

There is no objection to building an upper floor for the mosque. However, it is not permissible to separate it from the lower mosque in such a way that the worshippers on the upper floor cannot see the Imam, except when there is a need, provided that the worshippers are aware of the Imam's prayer, such as by hearing his voice or the voice of someone conveying on his behalf.