Blank or equal in data validation

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

Hello oteixeira,

I have used some helper columns on Sheet2 with Formulas, and Conditional Formatting on Sheet1, where you would be entering your Data.

I am lost in trying to summarize or explain in this post how to go about with what I have done. Sorry about that, but please try the attached Sample Workbook.

Thank you!

Hello oteixeira,

Oops! I forgot to work on the Colouring of Cells C3 and D3, to draw the users' attention, if Cell C3 is "Empty".

Please select those two Cells and set Colour to "No Fill", > go to Conditional Formatting, > Select "Manage Rules", > "New Rule", and enter the formula as shown below, and deicide on which "Fill Colour" you would prefer.

Formula:

=AND($C$3="",$D$3="")

Thank you.