It was hard to spot - but you should have it if you would use debug of the browser...
You have no reference to jquery.js in your page!!! jquery-ui.js requeires jquery.js to work!!!
My version of your sample code - works perfectly...
<!DOCTYPE html>
<html>
<head>
<title>Validating JQuery Date Less Than Selected Date</title>
<link href="Content/themes/base/jquery-ui.css" rel="stylesheet" />
<script src="Scripts/jquery-2.1.1.js"></script>
<script src="Scripts/jquery-ui-1.10.4.js"></script>
</head>
<body>
From: <input type="text" id="txtFromDate" />
To: <input type="text" id="txtToDate" />
<script type="text/javascript">
$(document).ready(function () {
$("#txtFromDate").datepicker({
numberOfMonths: 2, onSelect: function (selected) {
$("#txtToDate").datepicker("option", "minDate", selected)
}
});
$("#txtToDate").datepicker({
numberOfMonths: 2,
onSelect: function (selected) {
$("#txtFromDate").datepicker("option", "maxDate", selected)
}
});
});
</script>
</body>
</html>