JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.8.1/jquery.validate.js"></script>
<script src="http://ajax.aspnetcdn.com/ajax/mvc/3.0/jquery.validate.unobtrusive.min.js"></script>
<form method="POST" action="url" id="myform">
    <p>It is valid if fiels on the left contains the same value than field on the right</p>

    <input type="text" name="StartDate2" id="StartDate2" value="2"
           data-val="true" data-val-customequal="xx xxx" data-val-customequal-other="EndDate2"/>   
    <input type="text" name="EndDate2" id="EndDate2" value="3"
           data-val="true" data-val-customequal="xx xx" data-val-customequal-other="StartDate2"/> 
    
    <br/>
    <input type="button" value="Validate" />
</form>

CSS

.input-validation-error { background-color:Red;}

JavaScript

$.validator.addMethod("customequal-method", function (val, el, p) {
    
    var $other = $(el).closest('form').find('input[name=' + p.other + ']'); 
    return $other.length && $other.val() == val;
});

$.validator.unobtrusive.adapters.add("customequal", ["other"],
                                     function (options) {
                                         options.rules["customequal-method"] = options.params;
                                         options.messages["customequal-method"] = options.message;
                                     });



$(function(){
    $('#myform').data('validator',false);
    $.validator.unobtrusive.parse($('#myform'));
    $('[type=button]').click(function(e){e.preventDefault(); $('form').valid();});
    
    $('input[type=text]').blur();
})