The new 10/20/30 eTrexes can read multiple map .img files from internal memory or SD card. The setup map menu allows you to select which are displayed. If a map is transparent (eg. a contour overlay), or covers a completely different area to another map, they can be usefully enabled at the same time. Otherwise you just see whichever map is on top in the arbitrary rendering order.
You can do the same thing with a HCx eTrex (I don' t know about the Edge), but they only support a single GMAPSUPP.IMG file, which means you have to use Mapsource to combine the maps.