I have 2 copies of Super Mario 3D land, each game has one save on it, I want to know if it's possible for the saves to be decrypted and then combined into one save so I can then sell the left over game?

The save encryption key is unique per game card, not per game, so even if it was found for one person's game card it wouldn't work on all the rest unless the method to obtain the key could be reproduced.The save encryption key has not been found for SM3DL as far as I know, so no. You can't combine your saves.