Using Map Source I loaded the Idaho maps and Montana maps. Now on the GPS the Idaho maps overlap the Montana maps covering the Montana side so you can't see the Montana side. It is like the frame of the Idaho map is not transparent. Anyone see this before?
Yes, that is very common when using maps from different authors that were not designed to work together. Really, all you can do is to only use one map at a time if you have this problem. Some authors here have created mapsets for different states that were designed to work together. Most of them do not.
If you purchase Garmin's maps, they will be seamless and not has this problem.
As pointed out above, just disable the map on the GPS in the field you are not using. It is assumed that you are only in one of the two states at a time.