I was experimenting on downloading multiple maps on the card last night and it can be done. Each time I download a map I will get about 6 files that includes; 3 map files, an *.at file, a permit file, and the contour file. I found out I would have to load all of these for each area. For the files with similar names, I would have the PC number them 1,2,3 and so on.
On my Elite 7 HDI, only the downloaded maps would display on my chart; the base map is gone. This is kinda annoying since I can't keep the card in the unit and go speeding off to a different body of water.