What is the ruling on a worshiper joining one of two congregations praying at the same time in one mosque, separated by a wall?

It is not permissible to hold two congregational prayers in the same mosque at the same time, as this leads to disunity and confusion. However, if this happens unintentionally, then you should join the congregation with the better imam. For example, if the imam of one congregation is a reciter (of the Qur'an) and the other is illiterate or there is a disagreement about his imamate. If both congregations are equal in merit or their conditions are unknown, then join the one with the larger number, for the Prophet, peace and blessings be upon him, said: "Indeed, a man's prayer with another man is purer than his prayer alone, and his prayer with two men is purer than his prayer with one man. And whatever is greater in number is more beloved to Allah the Almighty."