Here Fiddle: http://jsfiddle.net/qndjW/772/
HTML:
<div class="wrapper">
<div id="content">
<h3>Main content</h3>
</div>
</div>
<div id="overlay">
<div class="wrapper">
<div id="sidebar">
<h3>Sidebar</h3>
</div>
<div id="leftsidebar">
<h3>Sidebar</h3>
</div>
</div>
</div>
CSS
#content {
background: #bbb;
width: 50%;
margin-left: 25%;
}
#overlay {
position: fixed;
top: 0;
width: 100%;
height: 100%;
}
#overlay .wrapper {
height: 100%;
}
#sidebar {
background: #ddd;
width: 25%;
float: right;
max-height: 100%;
}
#leftsidebar {
background: #ddd;
width: 25%;
float: left;
max-height: 100%;
}