2つのiframeを同期スクロールさせる #JavaScript

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
    </head>
    <body>
        <iframe id="iframeTxt1" src="longlong.txt"></iframe>
        <iframe id="iframeTxt2" src="longlong.txt"></iframe>
    <script>
        var iframe1 = document.getElementById("iframeTxt1");
        var iframe2 = document.getElementById("iframeTxt2");

        function scroll1() {
            iframe2.contentWindow.scrollTo(0,iframe1.contentWindow.pageYOffset);
        }
        function scroll2() {
            iframe1.contentWindow.scrollTo(0,iframe2.contentWindow.pageYOffset);
        }

        document.addEventListener('DOMContentLoaded', function() {
                iframe1.addEventListener('load', (function(element) {
                        return function() {
                                element.contentWindow.addEventListener('scroll', scroll1);
                        };
                })(iframe1, false));
                iframe2.addEventListener('load', (function(element) {
                        return function() {
                                element.contentWindow.addEventListener('scroll', scroll2);
                        };
                })(iframe2, false));
        }, false);
    </script>
    </body>
</html>
参考
https://qiita.com/sinsia/items/54865025821cb9203cfc