Is it permissible to hold two congregational prayers at the same time in one mosque, and if some young men pray in congregation before the regular Imam due to their busy schedules, should the Imam wait for them or establish his own congregational prayer?

If a mosque has a regular Imam, the Hanbalis consider it forbidden to establish another congregation before him. The Shafi'is, however, dislike this if the mosque is not frequently visited by people. After a congregation begins their prayer, the Imam should wait for them to finish their salutations, fearing that there might be two congregations in one mosque at the same time; because this is disliked, even though the prayer of both congregations is valid.