SEA-SEAsiaTopo Map

[sockmonkey]

Great, persistence paid off. 

As far as I know, only one mapset at a time.  You can do some overlays in several different formats, which it looks like what you have already done that were saved on your gps unit and then imported to basecamp.  As I was writing this I paused for a moment I opened base camp and then opened mapsource and did view two different maps.  So you might get a copy of mapsource https://www8.garmin.com/support/download_details.jsp?id=209

and see if that will work for you after resizing the windows.