If you compare the lighter (coverage) areas from the two bases, you will notice how the north and south base combination works. Although a coverage overlay gives a us a good idea of what to expect, in fact coverage is usually better than that due to signal reflections etc.
With your mouse OFF the map, you will see the coverage from the Barren Tier (Northern Lakes) base.
With your mouse hovered ON the map, you will see the coverage from the Bradys (Southern Lakes) base
Locate the place you want to go to and then move your mouse on and off the map to see if there is coverage there (the lightened areas).