JSFiddle - React, Tailwind, and code Playground
HTML
<select name="primero" id="primero" onChange="cambiaotro();">
<option value="0">0</option>
<option value="1">1</option>
</select>
<br>
<select name="segundo" id="segundo">
<option value="0">0</option>
<option value="1">1</option>
</select>JavaScript
function cambiaotro(){
var primero = document.getElementById('primero').value;
var segundo = document.getElementById("segundo");
segundo.value = primero;
}