JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<select id="select1">
  <option value="1">1day</option>
  <option value="2">2day</option>
  <option value="3">3day</option>
</select>

<select id="select2">
  <option value="1">1day</option>
  <option value="2">2day</option>
  <option value="3">3day</option>
</select>

JavaScript

$(function() { //run on document.ready
  $("#select2").change(function() { //this occurs when select 1 changes
    $("#select1").val($(this).val());
  });
});

$(function() { //run on document.ready
  $("#select1").change(function() { //this occurs when select 1 changes
    $("#select2").val($(this).val());
  });
});