All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You are not allowed to leave the obligatory Salah of Al-Fajr for the sake of observing the two recommended units before it. That is because when a person starts observing an obligatory prayer, he has to finish it. On that, the Kuwaiti Fiqh Encyclopedia states, “Scholars unanimously agree that it is impermissible to leave an obligatory act of worship after starting observing it for no valid legal excuse since doing so contradicts the sacredness of worship. Besides, we are prohibited from invalidating our worship. Allah, the Almighty, said, {… and do not invalidate your deeds.} [The Quran, 47:33]
Besides, you can make up these two recommended units after observing the obligatory prayer or even after sunrise – a ruling based on the Hadith of the Prophet “Whoever did not pray the two Rak'ah (before) Fajr then let him pray them after the sun has risen.”
Allah knows best.