What is the correct time for Imsak (abstinence from food and drink) given that there are two Fajr Adhans (the first) and one for Subh, and should Imsak be before the Fajr Adhan or before the Subh Adhan?

The legal time for abstaining from things that break the fast is at the rise of true dawn (Al-Fajr Al-Sadiq), and not before that. This is based on the words of Allah Almighty: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night] from the dawn," and the words of the Prophet, peace and blessings be upon him: "Indeed, Bilal calls the Adhan at night, so eat and drink until Ibn Umm Maktum calls the Adhan." A fasting person can eat and drink until he sees the true dawn or hears its Adhan. If he is unable to do so, he can imitate people of experience and the muezzins of mosques by relying on the last Adhan, which is after the rise of the true dawn, or on the calendar that specifies prayer times.