Blank or equal in data validation

[oteixeira]

Hello to all,
Please see attached spreadsheet.
Cells marked in yellow are filled by using values on data validation (values A and M).
My goal is to force validation on the yellow area so the following conditions are met:
1.Only one of the cells can be blank, not both.
2.The two cells cannot contain the same chosen value.
Many thanks for any kind help,
Octavio

I was reading my post again and of course both cells are blank before validation, so cell A2 can be painted to call the user attention...

Thank You