Why did tranquility descend upon the Prophet alone and not upon him and his companion?

Verse 40 of Surah (definition) At-Tawbah (definition) speaks about the story of the Prophet Muhammad (peace be upon him) and Abu Bakr As-Siddiq's migration to the Cave of Thawr, clarifying that Allah aided His Prophet when the disbelievers expelled him, so he was "the second of two in the cave, when he said to his companion, 'Do not grieve; indeed Allah is with us.'" Interpreters have differed on whom the pronoun refers to in the Almighty's statement: "Then Allah sent down His tranquility upon him," [9:40] with two opinions:

1. First Opinion: The pronoun refers to the Prophet (peace be upon him). This is the more common opinion among interpreters, and it is supported by the fact that the pronoun in His statement: "and strengthened him with hosts which you did not see" [9:40] refers to the Prophet, and there is no separation between the two pronouns.

2. Second Opinion: The pronoun refers to Abu Bakr As-Siddiq (may Allah be pleased with him). This has been narrated from Ibn Abbas and some of the Tabi'in, and some meticulous interpreters have favored it, arguing that grief and fear were Abu Bakr's share, and that tranquility was already present with the Prophet (peace be upon him) before that. They clarified that the Almighty's statement: "and strengthened him with hosts which you did not see" [9:40] might refer to subsequent events like the Battle of Badr, and is not necessarily linked to the descent of tranquility in the cave.

In any case, the descent of tranquility upon the Prophet (peace be upon him) does not prevent it from also descending upon Abu Bakr. Rather, the context of the verse indicates that tranquility was granted to both of them, and the singular pronoun refers to the Prophet to signify the unity and companionship between them, and that the Prophet's support encompasses his steadfast companion.