I contacted Kickstarter & they told me that backers can only purchase one pledge. However, the solution they suggested is relatively simple:
Just go to 'Manage My Pledge' and increase the pledge to the value of the item(s) you want.
Then message me telling me what it is extra you want,
and hey presto I'll add it to your order.
So for example say you want a signed Cd of the album which you have purchased for £15,
but you want to Pete & Terry to send 3 Christmas cards to various of your friends, just increase your pledge by 3x£10 (or 3x£12 if you are outside UK) & then message me to tell me what you want & I'll do the rest.
Hope that's clear - any problems or questions, just send me a message