Yes and no, to both questions.
With the exception of some recent models, a Garmin GPS can only access one map file, specifically named and specifically located on the SD card. I'm not sure whether the 605 qualifies as a 'recent model' but I suspect not. So you could put two maps on but the 2nd one would be of little practical use.
More sensible to use two SD cards with a different map on each (ie both named the same) - that works. Swapping the SD card will not affect your routes which are held in memory (even if they were originally loaded on the SD card - once the Route is made active, is is in memory, not on the card).
However you can also combine more than one map (eg City and Topo, or City, Contour map and OSM, whatever) into that single map file on the SD card - and the GPS will be able to display them separately or together, settable in the menus. This is very easy to do in Garmin's Mapsource software, and a very good reason to have it even if you don't use it for anything else.
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