I have just learned that I am carrying twin boys, Al-hamdulillah. Does that mean that four sheep have to be slaughtered for the Aqiqah--two for each boy?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If a woman gets twin sons, their father is recommended to slaughter two sheep of the same size for each one of them on the seventh day after their birth, i.e. four sheep for the two sons. Abu Hurairah reported that the Prophet said: "Each new born baby is hostage of his Aqiqa which is slaughtered on behalf of him on his seventh day and he is given a name (on that day) and his hair is cut". Narrated by Ahmad.
Ibn Qudama (a Muslim scholar) reported a consensus among all Muslim scholars that it is likable to slaughter the sheep on the seventh day and that it is likable to slaughter two sheep for the baby boy. There is no difference in this rule between a boy who was born separately or part of a twin because the rule is relative to the self. As for the girl, her Aqiqa is one sheep. If the woman delivers twin baby girls, two sheep are slaughtered for them.
For more details about the Aqiqa, please read Fatwa:
81275,81462 and 81526.
Allah knows best.