Textual: We see here another of the characteristics of Mormon's style. Verse 17 is a repeat of the basic information in verse 13. The essential information communitcated is that they separated.
When Mormon wrote, he had two tasks before him. The first was to construct the historical framework of his story, and the second was to hang the gospel onto that framework. In this case, Mormon presented both types of information in the same verse. He indicates that there is an event of structural import for the story; the Sons of Mosiah separate. This will become very important as the story of these missionaries unfolds, as Mormon will be relating stories from different protagonists. He is also interested in the gospel import of the story he is telling. Therefore, he organizes this section in the following way:
Mormon the editor takes his read down the structural path, diverts for a religious aside, and then returns to the historical backbone by repeating the point at which the diversion took place. We have seen Mormon do this on other occasions.