Yes! You can pledge for as many of a reward level as you like. Just adjust your pledge upward to account for the additional sets. For instance, if you would like to get two ADVENTURER rewards just back the campaign at the $45 pledge level shipment will be collected. Then, adjust your pledge to $45 and the extra shipping cost will be added when the campaign ends.
If you need to make this adjustment after the pledge has already been made, just click the "Manage Your Pledge" button on the Fantasydice Nightwatch Metal Dice Kickstarter page. Then click "Change Your Pledge" and Kickstarter will let you made the adjustment.