There is a great emphasis to join prayer in a first Jamaat .Granting permission for second jama'ats the door is opened for discord and disunity to set in, as any group that has slight differences with the Imam on any issue will then begin performing their own jama'at in the masjid or due to his lazy habit not to reach mosque on time.
Second Jamaat is permissible in certain conditions like for example those mosques where there isn't any fixed Imam to lead the prayer. Such mosques we find on the way while traveling where Muslims keep on coming and stopping by for prayer whenever they reach to the Mosque.
