Mormon gives few details about this particular Lamanite army, leaving many unanswered questions. Is this a coordinated effort from a single Lamanite commander, or is this a second army from a completely different Lamanite political unit? We simply don’t have enough information. I see two possibilities.
First, either the presence of a second army at the same location suggests that they are came from the same place and that there were two divisions (whatever military terminology might be used) that arrive at different times, or second, they were two independent armies who coincidentally arrived at the same time.
Arguing for the first possibility would be the timing itself. A coordinated attack is more likely to bring two armies together than random chance. Arguing for the second suggests that they simply met on the likeliest route from Lamanite territory into the Nephite valley. There is not enough evidence to choose between these alternatives.
Redaction: The appearance of this second army suggests that Zarahemla was experiencing two massive invasions within days of each other. Had Mormon written his text during the excitement of the moment, this second appearance would have been much more consequential than the brief summary he gives here. As a military man, he would have disposed quickly of the Amlicite/Lamanite curse and focused on the tremendous pressure on the Nephites to meet such large armies so rapidly.
However, Mormon is not writing a contemporary account. Although doubtless significant at the time, this invasion was something of a “blip” when seen in perspective. Furthermore, it is not really Mormon’s purpose to describe wars. Rather, Mormon is detailing the events leading to the Messiah’s coming among the Nephites; that birth is only eighty-seven years in the future. The narrative must account for the social changes effected between the relatively righteous Nephites of Alma’s day to those who are persecuting the believers in Nephi2’s (3 Ne. 1:9). These events are part of a spiritual war, not a physical one. That is Mormon’s reason for devoting so much space to the Amlicites’ inadvertent fulfillment of prophecy. It is the spiritual lessons of these wars that concern Mormon.