I tried to follow the tutorials/instructions and like to share the outcome of my experiments:
Device; Zumo 660 - firmware 4.30
microSD card SanDisk 16Gb class4, formatted FAT32
BaseCamp 3.2.2. on PC.
4 Map-Images: CityNavigator, 3x OnRoute (Walking/Bike/MotorBike), all unlocked.
The tutorial on displaying all Images in BaseCamp - succesful for each one as IMG (on SD \Garmin) with is't own name.
No difference for the location on microSD; either \Map\, or \Garmin\, or \Garmin\Map\
However in this way the maps don't show on the ZUMO (Settings/Map/Map Info).
They do show up, both in BaseCamp and the Device (each in turn) if the IMG file is renamed to "Gmapsupp.img" and placed in \Garmin\ on microSD.
No luck with naming a second map as Gmapsup2.img and placing it on SD in \Garmin, as suggested for other devices (Nuvi).
The other suggestion for a second map succeeded; naming it as "Gmapsupp.img" and placing this second "gmapsupp.img" directly on the device in \Garmin.
In this way both maps are visible in BaseCamp and in "Map Info" on the device both can be checked or unchecked.
Next I followed the tutorial for joining maps with GmapTool; the 4 Imagefiles together stayed beneath the 4Gb threshold (FAT32 file-limit).
The outputfile I named "Gmapsupp.img" and copied it to microSD in \Garmin\.
The result: 4 separate maps on the device, each of which can be checked/unchecked in "Map Info"
However in BaseCamp only 1 map seem to appear on SD; just CityNavigator. At closer look it's clear that the cartographic detail of this map is much greater than the original.
So the details for walking and biking are incorporated in the CityNavigator map in BaseCamp.
With these 4 maps on my device I can use it for different situations, either for walking, biking or for touring on the motorbike.
Thanks for all the useful tutorials/information.