Pivot Table should work, or you create a third page that sums the two together.
The third page consists of all individuals, this is perhaps more than 500?
Assuming your duplicate names are spelled the same.
Sheet1 is your first sheet
Sheet2 is your second sheet
Sheet3 is the combined Sheet1 and Sheet2
On the third sheet copy all the names from sheet1 and sheet2 paste in column A. Use the same titles and sort alphabetical and delete duplicates. Then in column D add this formula and copy down to the last name.
The sumproduct will update the two spreadsheets whether they are open or closed. If you never need to use those other two you can hardcode the new spreadsheet.
=SUMPRODUCT(--(TRIM(Sheet1!$A$1:$A$600)=TRIM(Sheet3!$A3)),--(Sheet1!$A$1:$A$600<>""),Sheet1!$D$1:$D$600)+SUMPRODUCT(--(TRIM(Sheet2!$A$1:$A$600)=TRIM(Sheet3!$A3)),--(Sheet2!$A$1:$A$600<>""),Sheet2!$D$1:$D$600)
