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: 
If A1<>"",B1=""
entered in one and 
If B1<>"",A1=""
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