Each card has a unique name. You can change that name prior to putting data/media on it by inserting the card into the device then tap card info, then menu and then Rename Card. Here is where you call it John2.
As soon as you hot sync - the Palm Desktop will sense that this is a different card and it will now show you a new Tab or Folder with the data that is contained on this card with the Tab name as the Card Name, typically. If you don't name the card then it will call it Expansion Card. It will keep adding cards as long as you do and it does not share this media until you tell it.
Media has it's own conduit and is dependent upon how you set it up. If you have the conduit set to sync and you delete a file from the Card - it should also delete it from the Palm Desktop upon next sync, which I just confirmed on a test.
But let me be clear on something - unless there is a conduit controlling the data - the contents of the SD card are not backedup. Sorry for that confusion.
But you should be able to add the second card with media controlling it so you don't get dupes unless something is a miss on you desktop.