Sheet 1 and Sheet 2
Sheet 1 cell A1 = 100
Sheet 2 cell A2 = 50
Sheet 2 cell type exactly this =A1+Sheet1!A1
Answer 150
Adjust as reqd
=SUM(D9+Sheet2!C7) OR WHATEVER CELLS CONTAIN THE TOTALS
You can also use the following format, if you want to say capture the total of cell A1 across sheets:
=SUM(Sheet1:Sheet2!A1)
Excel is pretty flexible at letting you point around to what you want to include in a formula, so if you are getting caught up on syntax, play around with it a little. Just be sure to hit an operater or enter after you've selected the cell you want to include.
good luck