JSFiddle - React, Tailwind, and code Playground

HTML

<select>
    <option>Choose</option>
    <option value="1">1</option>
    <option value="2">2</option>
</select>

<input type="radio" name="radio" value="1" />
<input type="radio" name="radio" value="2" />

JavaScript

(function() {
    var sel = document.querySelector("select"),
    opts = document.querySelectorAll("input[type='radio']");
    
    sel.addEventListener('change', function() {
       var val = this.options[this.selectedIndex].value;
       for(var i=0, max = opts.length; i<max; i++) {
           if (opts[i].value == val) {
               opts[i].checked = "checked";
           }
       }
    });
    
    for(var i=0, max = opts.length; i<max; i++) {
        opts[i].addEventListener('click', function() {
            var val = this.value;
            for (var i=0, max=sel.options.length; i<max; i++) {
                if (sel.options[i].value == val) {
                   sel.options[i].selected = "selected";
                   return;
               }
            }
        });
    }
})();