Years ago, i was able to find the map extention file of an old Nuvi 200, copied it to SD. If i wanted to use my old handheld 60csx for double duty while traveling, i could swap cards. (Both units, car and handheld, could only recognize a single map file at a time). The handheld could road navigate well running the Nuvi's map file card. It could not speak like the Nuvi car unit, but it did alert you to turns, etc with beeps.
I think new Garmin handhelds can recognize more than one map file. You just have to alter certain spelling.