There are two Hadiths that I recently came across:
'A'ishah reported: Ibn Umm Maktum was the mu'adhdhin of the Messenger of Allah (May peace be upon him) and he was blind. Sunan Abi Dawud 535
Narrated 'Abdullah bin 'Umar:
Allah's Messenger (ﷺ) said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan." Sahih al-Bukhari 620
Did Masjid e Nabawi have two Muezzins during the Prophet's lifetime simultaneously? If yes, why were two azaans given for Fajr prayer in Ramadan for the same jamaat?