gmapsupp.img files

Assuming you are using Mapsource, I think it's just a matter of selecting the required map tiles from both map sets and then transferring them as one entity. You may want to set the 'Map Set Name' manually to reflect the combined maps.

Ok, I am using mapsource, but the map tiles are of the same map, I only need the topo at certain times, because it is alittle off when driving down the interstate. I just bought nav NT 2009, but it was the full version,and I needed ( as I found out)just the update. I also found out you could only select map tiles from the NT09 version and load them to my SD card. So, I thought maybe there was a way I could select the map tiles which would be the same for the topo and still be able to select which one I wanted to use. Is this possible?

I think you're right, in that if you build GMAPSUPP.IMG by combining map tiles that overlap the same area, there's only going to be a single 'map' entry in the menu; rather one for City Navigator and one for the 'Topo'.

I don't think any filenames other than GMAPSUPP.IMG are supported on the SD card - so the only other option is to load one set to the internal memory...

I don't know the practicalities of doing that - but it is surely possible (Using UPDATER.EXE rather than Mapsource?)

You might need a reply from an actual C3xx owner ...

Possibly you can use two cards and switch when you need the other maps.

I don't know if this will help at all, but this is how I did it on my Kenwood DNX7100 that runs Garmin. I only use the topo when I go trail riding, but all i do is disable one and enable the other and that does the trick...here's my step by step of what I did:

http://www.poi-factory.com/node/17896

You should still be able to send them as one mapset and then you will be able to deselect the either mapset on the GPS unit on the Map Info screen. They should show up separately since they came from different map products.