The problem is that you have to download all the maps to your GPS unit all at once. Doing one map at a time will overwrite the other.
Using Mapsource, you have to copy and paste the trail map onto the City Navigator map and save it, then transfer it to the GPS. Once on your unit, click into your menu and you can check or uncheck the trail map so that it is only "on" when you want it.
Silly if you ask me, especially if you only want to add a couple of trails.