What is the legitimacy in Islam of a fighter's grant if the verification of his fighting was done by the testimony of two witnesses who heard but did not see?

The ruling on a grant depends on the conditions set by the granting authority, based on the Prophet’s saying, "Muslims are bound by their conditions." Based on your mention of requiring two witnesses to hear the news, if the hearing is widespread and common, then the testimony is valid, because "testimony based on hearsay" is acceptable and has detailed rulings in Islamic jurisprudence. Ibn al-Qasim mentioned that Malik considered testimony based on hearsay to be "a strong matter."