However, in the Pen, all I'm seeing is the square over which I hover turns purple. That is, .item:hover { background: blueviolet; } does what it's supposed to and that's it. I get the impression that what you expect to happen is hovering over box #1 in .grid will also change the background of box #1 in .grid2, but it doesn't. I tried it in Chrome and in Safari 10.1.1.
I assume this is because A) hovering over a label is not equivalent to hovering over its associated input and B) you can't hover over an element set to display: none; (as input is).
input { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0,0,0,0); border: 0; }
It's true you can't highlight an arbitrary element by hovering/focusing on another element.