JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
<head>
    <title>New</title>
    <meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
    <script>
      function event_1() {
        var text = document.forms['form1']['select1'];
        var text2 = document.forms['form1']['select2'];
        text.onchange = function() {
          text2.selectedIndex = text.selectedIndex;
        }
      }
    </script>
</head>
<body onload="event_1();">
    <form name="form1" onload="event();">
    <select name="select1">
        <option name="1">test1</option>
        <option name="2">test2</option>
        <option name="3">test3</option>        
    </select>
    <select name="select2">
        <option name="1">test1_2</option>
        <option name="2">test2_2</option>
        <option name="3">test3_2</option>       
    </select>
</form>
</body>
</html>