In fact, the game will automatically create an extra data if there isn't any found on your SD Card. It will be used to receive SpotPass data and temporarily store it until you play with one of your characters.
I don't believe you will need to swap SD Cards, because a 3DS game card carries every important data in it. This includes your town, progress, and even the DLCs you've received so far. There's no problem on using two 3DSes for the same game card.
I don't believe you will need to swap SD Cards, because a 3DS game card carries every important data in it. This includes your town, progress, and even the DLCs you've received so far. There's no problem on using two 3DSes for the same game card.