And the evidence for this is that the Prophet (peace be upon him) had ordered one of the two to give the adhan, and had not ordered both of them to give the adhan at the same time.

Sayyiduna Malik bin Huwairith (may Allah be pleased with him) narrates that we came to the service of the Messenger of Allah (peace and blessings be upon him) while we were young men of similar age. We stayed (with him) for twenty nights. And the Messenger of Allah (peace and blessings be upon him) was extremely kind and gentle. Then, when he (peace and blessings be upon him) sensed that we were longing for our families or eager to meet them, he asked us, "Whom have you left behind at your homes?" We informed the Prophet (peace and blessings be upon him), so he said: "Go back to your families, stay with them, teach them (the religion), and command them (to do good deeds)." And the Prophet (peace and blessings be upon him) mentioned some things (some of which I remember and some I do not), and said: "Pray as you have seen me praying. Then, when the time for prayer comes, let one of you give the call to prayer (adhan) for you, and let the eldest among you lead the prayer." Abdul Wahhab bin Abdul Majeed narrated as Bundar narrated, and at times differed with him in some words.

Abu Qilabah narrated from Malik bin Huwairith and reported the complete hadith.