Let's say, assuming the target range is A1 cell for each sheet.
The formula in A1 Sheet3 would be like this.
Excel Formula:
=IF(Sheet1!A1>Sheet2!A1,Sheet1!A1,Sheet2!A1)
Is one number always positive and one number always negative?
If not, I believe that there are some patterns as follows.
Pattern1:Sheet 1 has a positive number and Sheet 2 has a negative number.
The positive number to be in Sheet 3.
Pattern2:Sheet 1 has a positive number and Sheet 2 has a positive number.
Which positive number should be placed in Sheet 3?
Pattern3:Sheet 1 has a negative number and Sheet 2 has a negative number.
No number to be placed in Sheet 3. (Correct?)