بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
The second azaan that is done during the Jumuah prayer was established by Uthman bin Affan (RA) who was the third Caliph of Islam. He had established the second azaan due to the large quantity of Muslims that existed during his reign. This action of Uthman was accepted by the Ummah at that time and is done until today.
The Prophet (SAW) said The Prophet (peace and blessings be upon him) said, "Those of you who will outlive me will see many controversies, so follow my sunnah and that of the rightly-guided Caliphs." (Ibn Hibban and al-Hakim)
The second azaan in regards to its judicial ruling is a sunnah. This is based on the hadith mentioned above.
Fath al-Bari (2/394)
Abu Dawud #4607
Only Allah knows best
Written by Maulana Nabil Khan
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham