I remember that I came across the similar topic, but I can't find it now.
Is there a way to install and use several maps by garmin.openstreetmap.nl in Garmin Basecamp. It seems that the maps downloaded separately can't get along with each other, probably because they have the same name. In the topic I'm missing now it was advised to install Mac OS gmap set as it doesn't put any registry entries like Windows Installer does. I've tried and get no success. Basecamp still displays only one map. Renaming via JaVaWa GMTK is not a remedy either. The map gets a different name only visually, but seems still retaining original name in one of its technical files.And as I install another map set the previously installed map gets overwritten and non-readable for Basecamp.