What is the proper timing relationship between the pre-dawn Adhan and the Adhan marking the entry of time for Fajr?

Primary text

It is advisable that the pre-dawn call should not precede the actual time by too great an interval, lest the primary purpose of awakening people is missed. It is reported that there was an interval between Bilal's call and the call of Ibn Umm Maktum, allowing for one to descend while the other ascended. It is recommended that the Adhan not be called before Fajr unless there is another caller who calls upon the entry of the time. This practice follows the example of the Prophet, peace be upon him, and ensures that the intended announcement of the proper time is achieved by the second caller, with the first caller serving as a proximate warning.