What is the ruling on one who does not pray Jumu'ah in the Old Mosque but prays it in another mosque?

The fundamental principle is that there should not be multiple Friday prayers in one town unless there is a need, such as a long distance or a small mosque. If there are two mosques, one old and one new, and the old one is spacious enough for its congregation, then the valid prayer is in the old one. It is not permissible to have multiple Friday prayers in every mosque without a need. If the old mosque is not far from you and is spacious enough for you, then it is not permissible for you to pray in the new mosque. However, if it is far away by a difficult distance or cannot accommodate the worshippers, then there is no objection to praying in another mosque.