Unfortunately, Kickstarter doesn't allow you to make multiple pledges for multiple rewards. But, there are still two options to acquire multiple rewards:
1) You can increase your pledge amount to cover the additional cost of the extra tier and let us know what additional reward you'd like to receive, either by comments, messages, or through the survey we'll be sending out at the end of the campaign. We will be sure to make a note in our backer report about what the additional funds were for to ensure you receive both rewards. To increase your pledge, you just click the button to "manage" your pledge on the project page and increase the dollar amount without changing the reward tier.
2) You can pledge to our project as a “guest.” To do so, you should either pledge via an alternative browser where you aren't logged in or simply log-out and pledge again without logging into your Kickstarter account.