Maybe Jesus was referring to a process He knew would happen when people started to hear and agree on His teachings, becoming like-minded. Being as how the like-mindedness grew from the gospel He was teaching, He would recognize what would happen.
Perhaps Peter, being single eyed and loyal to the teachings, was the type of person that would form the core of those who would hear the gospel and come together. Perhaps Peter would evangelize creating a large number of like minded people.
The point is like-minded people come together of their likemindedness. Jesus would know full well how human beings work. We know human beings work like that because they do now, and have in recorded history.
Individuals come together and form a church "for, behold, the kingdom of God is within you" (Luke 17:21) and God is the guidance and fountain of knowledge and understanding for each person. Part of that guidance and understanding is seeking others who are brothers.