JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="eins" checked>
<input type="checkbox" name="zwei">

JavaScript

var eins = document.getElementsByName('eins')[0];
var zwei = document.getElementsByName('zwei')[0];

if(eins.checked) zwei.checked = true;

zwei.onchange = function () {
  eins.checked = this.checked;
};