Using a list, as Peo suggested, will force users to match the case used
in the list. For example, if the List is "Yes,No", you won't be able to
enter yes, or YES.
Another option is to Allow: Custom, and in the formula box, use an OR
formula to list the options. If the data validation is in cell D4 --
> I want a cell to allow only 2 valid values but not have a drop-down list
> How do I do that with Data->Validation?
Excel FAQ, Tips & Book List