Edit in JSFiddle

<div class="m-center">中</div>
<div class="left">
    <div class="main">左</div>
</div>
<div class="right">
    <div class="main">右</div>
</div>

html, body {
    margin:0;
    padding: 0;
}
.main, .m-center {
    height: 200px;
    line-height:200px;
    text-align:center;
}
.m-center {
    z-index: 2;
    background-color: #666;
    width: 500px;
    margin-left: -250px;
    position: absolute;
    top: 0;
    left: 50%;
}
.left, .right {
    z-index: 1;
    position: absolute;
    top: 0;
    width: 50%;
}
.left {
    left: 0;
}
.left .main {
    margin-right: 250px;
    background-color: #ccc;
}
.right {
    right: 0;
}
.right .main {
    margin-left: 250px;
    background-color: #ccc;
}