Unfortunately, Kickstarter doesn't allow backers to pledge more than once. But you can change your pledge at anytime until the end date (May 20th).
I have added a pledge level (limited to 25 packages) for 2 signed copies of the book, with a special discount.
Otherwise, you have two ways of pledging for 2 books:
1- use two different credit card / accounts and pledge twice
2- At the top right of the page, there is a box where you can enter your own customized amount. Enter the amount for 2 books + shipping, and send me a message so I can make a note in my system that you paid for two books. For reference, these are the amounts:
--- 2 copies, US shipping = $170
--- 2 signed copies, US shipping = $190
--- 2 copies, International shipping = $190
--- 2 signed copies, international shipping = $210