Alma had preached twice and failed twice. The solution to Yahweh’s problem of sending a servant whom the people ignored was to call a companion. It is this pairing of Alma with Amulek that becomes the basis for the stories that Mormon chooses to include.
The addition of Amulek was not because of any incapacity in Alma. He was a tested prophet with an unshakable testimony who stood worthily at the head of the church. What did Amulek bring to their partnership? First, he was local man; and second, he also had tremendous faith. While Alma might be dismissed as a stranger whose belief may have been “put on” with his title, Amulek was among his neighbors. His belief could not be dismissed as either foreign or insincere. He had social connections that Alma lacked, and thus he increased Alma’s effectiveness. Together, Alma and Amulek became more powerful than either one by himself.