Give this a try. You will do this one at a time for each column... we'll do
Column A first.
If you want this functionality for the entire column, then select the entire
Column A; otherwise select all of the cells in Column A that you will want
to have this functionality and note the active cell within that selection
(I'll assume it to be A1). Now, click Data/Validation from the menu bar. On
the Settings tab, select "Custom" from the "Allow" drop down; then put the
following formula in the "Formula" field...
=LEN(B1)=0
Note that if you used a partial selection of Column A and the active cell
was not A1, then you need to use the row number from that active cell in the
above formula (that is, if the active cell was A7, the use =LEN(B7)=0 in the
formula instead). Also make sure the "Ignore blank" check box is checked.
While the dialog box is up, you can customize the message that appears if
the corresponding cell in Column B has a value in it via the "Error Alert"
tab. The "Input Message" tab is used if you want to display a message the
validation cell is selected.
Now, repeat the above procedure for Column B, although you will use this
formula for it...
=LEN(A1)=0
