I'm sure somebody who knows what they are talking about will be along soon, but until then
I don't know whether you GPS supports multiple .img files or not. They way to find out would be to hook it up to a computer and go exploring. Rename the gmapsupp.img file to another.img filename and then load some different maps (which will recreate the gmapsupp.img). Now turn on the GPS and see if all the maps show up. If not, then you can probably only use one gmapsupp.img file.
If you can only use one map, perhaps you can load the other .img into Mapsource and then select all the maps and send it to the GPS (effectively merging the multiple maps together into one gmapsupp.img).
Here's some instructions from the web http://www.gpsfiledepot.com/tutorial...mapsettoolkit/
Or wait until somebody who knows what they are doing comes along.