Since the one elected was being chosen to be an Apostle of Christ, the prayer asking that the chosen one be indicated is also addressed to Him.
As during His life on earth the Lord chose Apostles for Himself, so also now He must Himself choose the twelfth Apostle. For He had ascended into heaven, yet promised always to remain in His Church (cf. Chrysostom).
"Knower of hearts" means the One who knows the human soul in all its inward hidden places. Therefore He fully knows which of the two previously chosen men is more capable of bearing the ministry laid upon him.
Peter had firmly expressed his faith in the Lord's omniscience and knowledge of hearts already during the Lord's life with the Apostles.
He did this especially in the significant conversation with Him after His Resurrection, on the shore of the Sea of Galilee (Jn 21:15 and following).
In another speech Peter calls God the Knower of hearts (Acts 15:7 and following). Therefore, by now applying this word to Christ, he expresses a confession of His Godhead.
"Show" means by means of the lot (v. 26). "They did not say, Choose, but, Show the chosen one. They knew that with God all things are determined beforehand" (Chrysostom; cf. Theophylact).