Have you compiled maps with mkgmap? You need different “mapname” for each tile. If you have used default value for “mapname” option, then your GPS ignore second map, because it treat it as a second copy of the same data.
If you have downloaded custom maps from garmin.openstreetmap.nl you need to adjust the family id because both get the same id (2000 i think). You can do this with http://www.javawa.nl/jdm_en.html