Format-> Conditional Formatting is your friend
The easiest thing to do is to select the first box (B2 if I'm reading your
cheat sheet right) and do the following
Format-> Conditional Formatting
Choose Cell Value Is from the first drop down
Choose Equal To from the second drop down
Type "=B$1" in
Checkboxes can have a cell link (if you use the "forms" check box) that you
can then put a formula in
=if(and(a1<>"",b1<>""),True,False)
in the cell that the check box links to will do EXACTLY what you want.
Unfortunately, this will get overridden if the user ever checks or unchecks
the check box
