will try and report result ;-)
funny thing though (as feedback to the forum threads): i tried another solution; i used mapinstaller to build an IMG of all the countries NOT included in the built-in CE map and loaded the IMG to SD; it works, no conflict between GMAPPROM and GMAPSUPP, all cards show on the screen, so cnfhbr was right in his assumption that it is not an issue with the registration and unlock codes - interessting :-)
another funny thing - i did NOT load any Europe JCV files to the .sys nor SD Card, i just use the built-in CE JVC files; amazing finding: while simulating a route from Netherlands to UK, UK junction views appear although CE map does NOT cover UK ? wow ... - what is your opinion to that ? hidden GARMIN feature ?