If you have ever wondered how the map naming scheme works on Garmin devices it is really simple and can be used as a way to avoid merging maps.
Here are the map file names usable on a garmin device or simply put the GPS internal map names
gmapprom.img – Primary/Pre Installed Maps
gmaprom1.img – Secondary Mapset
gmapbmap.img – The base map
gmapoem – OEM Maps (cannot be deselected from device if used)
gmapsupp.img – Mapsets installed via Mapsource/Mapinstall
The SD or Micro SD card for a garmin device can only contain gmapsupp.img
This Screenshot shows an example of multiples maps on a single device using the naming conventions for garmin maps.
From what I can tell garmin map names are the same across all the nuvi devices I have looked at and it seems to be the best way to handle multiple garmin maps.