The solution to God’s problem of sending a servant who was not heard was to call a companion. Alma had preached and failed twice. The “cure” was to call Amulek to a mission along with Alma. It is this pairing of Alma with Amulek that becomes the basis for the stories that are included by Mormon.
Why did the Lord require Amulek in addition to Alma? Was Alma insufficient? Hardly. We have in Alma a tested prophet with an unshakeable testimony and a calling as the leader of the church in the land. What need did the prophet have of a companion?
Amulek was a local man, and one who also had tremendous faith. While Alma might be dismissed as one who came from another place, and who said he believed because he had to (he was, after all, the leader of the church) Amulek was in a completely different position in Ammonihah. He was known there, and his belief could not be dismissed as foreign. Amulek brought with him social connections that Alma was lacking, and provided to Alma capabilities that clearly increased Alma’s effectiveness. There are any number of times when the Lord chooses to expand his “army” and calls many into service. The leaders are not the only ones with testimony, nor the only ones capable of powerfully bearing that testimony. The Lord calls Amulek, and it is the pairing that become legendarily powerful – moreso than just one of the men by themselves.