What is the wisdom behind the Prophet (peace be upon him) being mentioned as "the second of two" in verse 40 of Surat At-Tawbah?

"Second of two" means "one of two," and they are the Prophet, peace and blessings be upon him, and Abu Bakr. The second was not mentioned because he was known and as an exaltation of victory despite the small number, for Allah is capable of granting victory in the future just as He granted it in the past.