The sons of Mosiah began their mission with the Lamanites. They separated. As you can see in the PDF, Ammon and Aaron had two very different experiences. Ammon immediately began to serve. In contrast, Aaron immediately began to preach.
If you remember the timeline, you realize that Ammon has great success and Aaron ends up in prison. Ammon (and Ammon’s new convert, King Lamoni) help rescue Aaron. Then Ammon and Aaron spend a short time together. One might imagine that Ammon explained how he had so much success.(SERVICE!) After that, Aaron heads over to King Lamoni’s father (the one that tried to kill Ammon). The first thing Aaron says when he gets there is “we will be thy servants.” King Lamoni’s father starts asking questions, Aaron begins to teach and the rest is history. Service is truly powerful!