Allow entry into a cell only if another is empty

[weh2oski]

I need to allow entry into cell A1 only if Cell A2 is blank or oposite. leave user of spreadsheet with choice of cells to enter but not allow entry into both.

 

[jimboy]

select cell A1, go to data > validation

select custom, in formula type;

=IF(A2="",TRUE,FALSE)

and in A2 do the same but change the formula to;
=IF(A1="",TRUE,FALSE)