It depends on what GPS you have. If you are using one that has internal memory as well as a memory card, then you can place NW Trails in the internal memory. Also, if you have a Colorado, Oregon or Dakota (and possibly other models, although I'm not sure which) and have the latest firmware update, you can rename the image files on the unit and still have the unit recognize them. Just be aware that when uploading maps, the default name given to the image file is "gmapsupp.img", so if a file with that name already exists in the destination location, it will be overwritten.
I have the 60cx, so I can't do multiple maps.. The big thing is to use the NWtrails on top of the garmin topo, that would be nice, but I would need to load the garmin topo into mapsource,which I can not do. I thought I read somewhere that someone was able to convert an IMG file (what garmin gives me) into a mapsource readable map..
Oh well, I thought I would float it and see what I got..