What is the evidence for prohibiting the establishment of two congregational prayers in one mosque, and is it permissible for the Imam to begin the Tarawih prayer while another congregation is praying the 'Isha' prayer?

If a mosque does not have a regular Imam, there is no harm in holding a second congregational prayer for the same Salah (definition). However, if it does have a regular Imam, the majority of scholars discourage repeating the prayer in congregation. Some scholars permit it, and this latter opinion is the more sound one. It is preferable for the Imam not to begin the Tarawih prayer before all worshippers have completed their prayers. However, if there is a need to hasten his initiation (of Tarawih), there is no harm, and the worshippers should lower their recitation. If the Imam begins Tarawih before they have begun their Ishaa prayer, they may follow him, as some scholars consider the obligatory prayer behind a supererogatory one to be valid, and this is the more sound opinion.