I don't think it's possible to achieve your aims in CSS alone. However, as other contributors have mentioned, it's easy enough to do in JQuery. Here's how I've done it:
HTML
<div
id="toplayer"
class="layer"
style="
z-index: 20;
pointer-events: none;
background-color: white;
display: none;
"
>
Top layer
</div>
<div id="bottomlayer" class="layer" style="z-index: 10">Bottom layer</div>
CSS (unchanged)
.layer {
position:absolute;
top:0px;
left:0px;
height:400px;
width:400px;
}
JQuery
$(document).ready(function(){
$("#bottomlayer").hover(
function() {
$("#toplayer").css("display", "block");
},
function() {
$("#toplayer").css("display", "none");
}
);
});
Here's the JSFiddle: http://www.jsfiddle.net/ReZ9M