|Use the same rule, simply select column C in the "Applies to" section.|
Or if you want to start over again:
1) Select your cell or range of cells: B1 : C100
2) On the ribbon click Conditional Formatting
3) Click on New Rules, it’s near the bottom of the dialog box.
4) Click Use Formula to determine which cells to format.
5) Enter the formula:
6) Click on the Format button
7) Select the Fill Tab
8) Select a pretty color
9) Click OK
10) Click OK
Note the Dollar Sign next to Column B in the formula,
it is necessary if you want both cells to turn color.
I only indicated 100 rows, B1:C100, change to suit your needs.
See how that works for you.