easy rider, your device support only one gmapsupp.img file with max size 4GB on micro SD (size of microSD may be up to 32 Gb and microSD must have HC specification only). You can use Mapsource but some Garmin maps format like BlueChart G2 and G2 Vision do not have installer for Mapsource. Advanced user may to make Mapsource installation from any maps but you must read this forum from from beginning to end to become it.
In you case you must use gmaptool to combine maps simply...