Hello, I have has this question pop up a few times now! I have checked and there are two easy ways to do this:
1) When you are about to purchase you can put in an extra amount, so you can type in £20 instead of the automatic £10
2)You can also change your pledge if you go back to the main Kickstarter page a box at the top will pop up saying 'manage pledge' if you click change pledge it will give you the option then to add extra money on top if you add an extra 10 on there that gets you a second book!
There are also other ways but these two are the easiest that I found!
Once the Kickstarter has finished I will be messaging everyone who has pledged more than the set amounts anyway making sure they want more than one book too :)