When you create the conditions in conditional formatting, put the
tougher test in the first condition. In your example, 'greater than 1.7'
is the tougher test. Numbers that don't pass that test will go through
the next condition -- greater than .8
Select the cell
Choose Format>Conditional Formatting
Leave the first dropdown as 'Cell Value Is'
From the second dropdown, choose 'greater than'
In the text box, type: 1.7
Click the Format button, and choose Red on the Patterns tab
Click OK
Click Add
Leave the first dropdown as 'Cell Value Is'
From the second dropdown, choose 'greater than'
In the text box, type: .8
Click the Format button, and choose Yellow on the Patterns tab
Click OK, click OK