The SD cards do not have to be the same in each camera that you're using in the 3D HERO System.  You can have different sizes, class ratings, and brand SD cards, and the sync will still be effective.  We recommend only purchasing name brand SD cards through reputable resellers.  There are both counterfeit and underperforming SD cards out in the marketplace, which can prevent proper functioning of your HD HERO camera's features..  For the HD HERO Original camera, we recommend 16GB Class 4 SDHC cards.  For the HD HERO2 camera, you can also use 16GB Class 4 SDHC cards.  If you want to take advantage of the 0.5 sec time-lapse and 10 photo burst modes, we recommend 16 GB Class 10 SDHC cards.  The largest cards that can be used with the cameras are 32GB Glass 10 SDHC cards.