Web Forms :: Textbox Validation Code Doesn't Run When Textboxes Are Empty?

I want the validation condition to be: on a pair of textboxes, to be valid as long as at least one of them isn't empty.

So I put a customvalidator on the 2nd textbox, hooked up to the following server code and client code:

<script type="text/javascript">
function ECPhoneOrEmailRequired(source, args) {
var ecPhone = document.getElementById("txtEmergencyPhone");
var ecEmail = document.getElementById("txtEmergencyEmail");
if (ecPhone.value.length < 1 && ecEmail.value.length < 1) {
args.IsValid = false;
}
else {
args.IsValid = true
}
}
</script>
Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs)
'args.IsValid
If (txtEmergencyPhone.Text = String.Empty And txtEmergencyEmail.Text = String.Empty) Then
args.IsValid = False
Else
args.IsValid = True
End If
End Sub

From what I can tell thru breakpoints, this code works fine as long as there's something in one of the two textboxes. When both textboxes are empty, the code is never called - and so both boxes empty ends up being allowed.

So what's the correct way to validate one-or-the-other-nonempty condition?