<style>
.container{
position:relative;
}
.div1 {
z-index:80;
width:500px;
height:500px;
float: left;
box-shadow: 0px 0px 80px #A0A0A0;
-webkit-box-shadow: 0px 0px 80px #aaa;
-moz-box-shadow: 0px 0px 80px #A0A0A0;
}
.div2 {
z-index:-10;
position:absolute;
width:500px;
height:500px;
float: left;
left:560px;
top:30px;
box-shadow: 0px 0px 80px #A0A0A0;
-webkit-box-shadow: 0px 0px 80px #333;
-moz-box-shadow: 0px 0px 80px #A0A0A0;
}
</style>
<div class="container">
<div class="div1">asdf </div>
<div class="div2">asd asdf</div>
</div>
z-index , div , , , , , .