I need to prevent the user from entering data in cells A1 & B1 on the same row. They need to use one or the other. For instance on row 1 they may need to use A1 therefore they cannot use B1, then on row 2 they may need to use B2 therefore they cannot use A2. If they use both cells on the same row the formulas dont work properly in other cells dependent on these cells. I need it to work like this all the way down the spreadsheet.

Thanks for the help