You can Union them in the Custom SQL dialog, or you can create an extract the September 2013 file, then add the October records using the "Add Data from File" option: Adding Rows from a File
I am assuming the columns/fields in both sheets are the same. There may be other suggestions.
Your Union option is assuming both sheets are in a same excel file, correct.
Yes, I believe so.
With Matt's solution, just combine the two sheets in to one excel file.
If combining them is not an option you can save them as CSVs.
If you save the two sheets as CSV's you can treat them as flat files and a then a UNION should be possible.