§1. Where it is possible and expedient, there is to be a major seminary in every diocese; otherwise, the students who are preparing for the sacred ministries are to be entrusted to another seminary, or an interdiocesan seminary is to be erected.
§2. An interdiocesan seminary is not to be erected unless the conference of bishops, if the seminary is for its entire territory, or the bishops involved have obtained the prior confirmation of the Apostolic See for both the erection of the seminary and its statutes.
Source: Code of Canon Law (Vatican.va)