I have never found a way to create a new pivot table using more than 1 pivot table as input. If you want to combine, you have to go back to the source data & make a whole new pivot. If the data lives on multiple sheets or is too large to fit on a single sheet, you run into problems.
I have a workbook that I use to work around this problem. If you have macros enabled, it will create a temporary menu bar item (on Excel 2007 it appears under Add-ins) that will allow you to select as many workbooks as you want as input to a single consolidated pivot table. The pivot doesn't have any default layout unless you specify the fields you want in the parts of the setup sheet labeled appropriately. Hopefully it's pretty self explanatory, I've given this away to various people, mostly to help them get around the 65,000 row limit in pre-2007 versions of Excel. I have used this to create a pivot from a combined > 550,000 rows of data. Maybe you can use it.
The thing I like about it is that you can create multiple pivots from the same source data - all the pivots created using the Create Pivot macro share the same pivot cache. Once the first pivot table is created, making more pivot tables results in very small file size increases.