You need to select all the map segments you want of city select, then in mapsource in the drop down menu(upper left) change you mapset to inland lakes and select the mapsets you want, then upload both at once.

Switching mapsets in the gps is kinda a pain…. from the map screen hit menu & select setup map. Now highlight the I-icon(map setup information), you can hide individual map segments here, but if you press menu again…. now you can show and hide whole mapset in this menu.