JSFiddle - React, Tailwind, and code Playground

HTML

<iframe src="/echo/html" id="iframe1" class="containerFrame"></iframe>
<iframe src="/echo/html" id="iframe2" class="containerFrame"></iframe>

<script>
    //ignore this script, its here just to put the textareas in the iframes....
    iframe1.onload = function() {
        iframe1.contentWindow.document.body.innerHTML = '<textarea></textarea>';
    }
    iframe2.onload = function() {
        iframe2.contentWindow.document.body.innerHTML = '<textarea></textarea>';
    }
</script>

JavaScript

$(function() {
    $('.containerFrame').each(function() {
        var frame = this;
        
        $(frame.contentWindow.document.body).on('input', 'textarea', function() {
            var textarea = $(this);
            $(".containerFrame").not(frame).each(function() {
                $('textarea', this.contentWindow.document).val(textarea.val());
            });
        });
    })
})