I have successfully used the following (assuming you know how to use GMapTool with cgpsmapper):

With a gmapsupp.img file from OSM, I used GMapTool to split it (rename it as 12345678.img first) and chose an arbitrary FID number (I started with 6000). Then go to the resulting output folder and run install.bat which must be Run as Administrator.

Then with a second gmapsupp.img file rename it to 12345678.img and then run GMapTool and choose a different FID number (e.g 6001) and output folder. Again go to the resulting output folder and run install.bat as Administrator.

Both OSM maps turn up in Mapsource (I am using v6.16.2) and in BaseCamp (v4.1.1.0). As pointed out in other comments, it is necessary to have the different FID numbers and that is best done by not using the Windows generic installer version for Mapsource from OSM.

Worked perfectly…