Please excuse me, but I was confused by your first post that says one of the validators fire and the other one not.
I believe in your case both of the validators are not firing. And this is because you do request when the view is changed, without actually click any button.
The buttons could have CausesValidation="true/false", and in case the property is false, they do not perform validation. If you however perform Ajax or PostBack manually, you will need to call Page_IsValid
manually, because it is equal to performing callback from button that does not have CausesValidation set to true.
The issue is not particually related with the RadDatePicker, you will get the same behavior with asp:TextBox as well.
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