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.
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.