Since Kickstarter will only allow you to choose one tier, you can add multiple items from different tiers by simply adding them together and pledge that total in one tier.
For example: If you want an item from tier one and one from tier 2, simply add the two totals together and pledge this amount in tier 1. At the end of the campaign, you will be sent a survey that will ask you which items you're paying for. At this time you can tell us you wanted 1 item from tier A and 1 item from tier B. No worries!