Yes you can. If you want 2 books instead of one, or 2 shirts. We can totally do that. It is simple, just pick your base reward and add the item cost per unit to your pledge amount.
Book - $20
Sketched Edition Book - $30
Shirt - $25
So let's say you want to buy 2 sketch edition books. Select the $35 tier and in the amount add $30 to your total for a grand total of $65. Don't select the $65 tier, just select $35 and add the amount. But if you select anything else on accident, it is no big deal. Just email me that you wanted more items and let me know how many and I will be sure that it all gets out to you.