What is the ruling on establishing two congregations in one mosque at the same time?

The student : We repeat the question... A mosque has regular worshippers, meaning people from the local area, and passersby. When it is time for prayer, the regular imam was delayed in coming out, so the iqamah was called and one of them led the prayer. The people of the mosque, the locals, waited for the regular imam while the people were praying. When the regular imam arrived, a second prayer was established, and they prayed as two congregations in the mosque at the same time, meaning some may have been late by one or two rak'ahs.

The Shaykh : The answer is that the first congregational prayer is valid; it is not said to be invalid, but it is a sinful congregation in another respect; because it started without permission and encroached upon the right of the imam. However, the imam also made a mistake when he should have been patient until this congregation dispersed, bearing their sins, and he should have established his prescribed congregation. As for countering the first congregation on the grounds that it was encroaching, he is the one encroaching upon it, and it is an established congregation. So this is permissible, but according to the school of Abu Nuwas, who said, "I treated the disease with what was the cause of the disease."
The student : Okay, on the same topic, is it better, O Shaykh, for him to follow the first congregation rather than wait?

The Shaykh : No, because this congregation is encroaching, so he should not follow it.