Combining several maps, from varied sources, covering disparate or overlapping geographies, into a single .img file is simple to do within good old Mapsource.
Where maps within a single .img overlap they can still be switched on and off individually, within the GPS device (thinking of an old Etrex for example).
Be interested in how to do this - for future reference. 
Sorry, missed that. It's as simple as point-and-click.
Various map products from Garmin (at least, those of a certain age) and downloadable map images from eg OSM can all be installed in Mapsource and become available under View/Switch to Product or from a dropdown list in the toolbar. You can for example have a Route or Track loaded ans simply switch between maps in this way.
Whichever map is displayed, you can select multiple map tiles using Tools/Map or the Map Tool in the toolbar. Your selections don't have to be contiguous. You can then export the selected mapset to your (old) Garmin using Transfer/Send to Device. If its a big mapset it takes forever (USB 1). Mapsource plays best with old Etrexes I find, so I have an old Vista on the desk that I use for this and nothing else. The gmapsupp.img file once transferred can be used in any other Garmin, and can, depending on model, be renamed.
With some map tiles already selected, you can switch to another map and continue to select tiles. These can overlap your existing selections (which you can't now see, but they are still selected) - eg, UK contours, or UK OSM vs UK Metroguide, etc. Or they can be in completely different parts of the world - eg, UK Garmin mapping around home and Heathrow, then OSM mapping in India. You can swap maps and select tiles as much as you like, all selections are retained. You can even save the total multi-layer selection as a GDB file if you wish (this is not essential). You can then export the selected (combined) mapset to your (old) Garmin using Transfer/Send to Device. There are limits, on total size and number of tiles, but they are very high and you are very unlikely to encounter them, given the slow speed of transfer you'd probably die of old age first.
On the Garmin, the imported map data is a single gmapsupp.img file. On older models, you are limited to this one file, on newer models you can have multiple .img files by renaming them. Where maps overlap, how they display depends on how they were compiled in the first place (there is a stacking order, and a transparency property, neither of which you have any control over by this stage) and on the peculiarities of your model of Garmin. But on old Etrexes that can only use a single .img file, individual map tiles can be shown or hidden, and tiles also group by map name and can be shown/hidden in this way. on newer Etrexes none of this is necesary - you just use lots of single-map .img files instead.
