I have always read that it is not safe and a bad idea to stack blocks. If you need to raise your rear by 1 inch, your supposed to buy one block that is 1 inch taller then your current block. Or, an add-a-leaf will work too.
Do not stack blocks. But just to throw it out there, Auto Zone has 1" blocks for $25. If you have a spring shop anywhere near you, they will have blocks 1" taller than your current ones and longer U bolts for ya too. good luck!