The judges of each United States district court and the district courts of the Virgin Islands, Guam, and the Northern Mariana Islands shall appoint United States magistrate judges in such numbers and to serve at such locations within the judicial districts as the Judicial Conference may determine under this chapter. In the case of a magistrate judge appointed by the district court of the Virgin Islands, Guam, or the Northern Mariana Islands, this chapter shall apply as though the court appointing such a magistrate judge were a United States district court. Where there is more than one judge of a district court, the appointment, whether an original appointment or a reappointment, shall be by the concurrence of a majority of all the judges of such district court, and when there is no such concurrence, then by the chief judge. Where the conference deems it desirable, a magistrate judge may be designated to serve in one or more districts adjoining the district for which he is appointed. Such a designation shall be made by the concurrence of a majority of the judges of each of the district courts involved and shall specify the duties to be performed by the magistrate judge in the adjoining district or districts.