If he only relays the takbeer of the Imam because he is up front and can hear him, this is permissible. But if he is offering a new prayer where he is the Imam, this is invalid as there is an ongoing congregation and it is the original.

Recently our masjid has been under development and the people on the second floor can’t hear the microphone to join the congregation salah, so a second person leads the prayer upstairs at the same time. Is this permissible? It’s caused some problems with some people due to this.