Data Validation Conditional Formatting allow entry into only one of two cells

I'm trying to format adjacent cells so that data can only be entered into either one of them, but not both. I've tried Data validation but can't come up with a custom formula that doesn't result in "Stop" alerts whenever either cell is selected. (For example:

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entered in one and

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entered in the other.)

Is there a way to disallow entry into both? Is there a formula that I can apply to both as a range using Conditional Formatting? I have long columns where each pair of these adjacent cells needs to be formatted per above.

BTW, I need to avoid VBA with this tool.

Thanks for any insights! David

Works perfectly. and so simple! YOU are my hero. Thanks. David