What is the ruling for holding two congregational prayers concurrently in the same mosque?

Holding two congregational prayers concurrently in the same mosque is one of the issues that causes problems among people. The ruling differs according to whether the mosque has an appointed imam; it causes fitna, and so forth. It is impossible to hold a second group prayer simultaneously with the first with the intention of causing division among Muslims when the mosque has an appointed imam and mu`ezzin (person who makes the call for prayers). This is prohibited and considered a sin although the prayer of the congregants is still valid. If the second group prayer is not intended to cause dissension and the congregants were unaware of the first group, the second congregation is valid and does not entail any sin. It is preferable to seek that which unites Muslims and not that which causes their division.
Allah the Almighty knows best.