I hike and hunt along the border of Montana and Idaho. Is there any way to combine both maps into one in base camp?
Not really, BaseCamp only allows a single map displayed at a time. But the maps would need to be made with that in mind with the data cut to the state border, and they were not.
You could always follow the map making tutorial and make your own that contains the area you hunt, or us a map that contains the contiguous data such as NorthWest TOPO:
http://www.gpsfiledepot.com/maps/view/412/