I may be missing something. Is this just a case of getting data from 2nd spreadsheet?
If so, just reference the 2nd spreadsheet properly. Example:
On spreadsheet that has JulyAug data … select the cell where you want the Sept data to populate & then type =[Sept.xlsx]Sheet1!$B$2
This is on Excel 2007 (should be same for Excel 2003 except .xls) Sept.xlsx is the 2nd spreadsheet & the data resides at cell B2.
If you want the sum of Sept data (spanning from B2 to B20) then click on AugSept spreadsheet where you want the sum … =Sum([Sept.xlsx]Sheet1!$B$2:$B$20)
If you have more complex situation that I did not comprehend … please give an example.