I made two separated files for each Topo US because combined .img exceeded 4 Go.
I notice the same problem if I put in the same garmin folder img file coming from Topo US South Central and another one coming from a part of Topo US North Central (North/South Dakota,Minnesota,Nebraska,Iowa) : only one of both maps is displayed on my Montana.
There are very strange phenomena about this topo US serial. May be somebody else already noticed that on Montana or on others GPS.