I have an inventory spreadsheet with three pages. Is there a way to
merge two copies of the spreadsheet such that the resulting spreadsheet
will have the sum all of the inventory totals in both copies without
changing anything else?

For example:

Spreadsheet 1:
1234 Item1 250 2345 Item2 39
3456 Item3 900 4567 Item4 22

Spreadsheet 2:
1234 Item1 75 2345 Item2 27
3456 Item3 450 4567 Item4 13

Merged Spreadsheet:
1234 Item1 325 2345 Item2 66
3456 Item3 1350 4567 Item4 35

Thanks for your time.