The following code shows how to nested span6 cell.
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" type="text/css"
href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.1.1/css/bootstrap.min.css">
<style type='text/css'>
.red {<!-- w ww . j a v a 2s . c o m-->
background-color: red
}
.blue {
background-color: blue
}
</style>
</head>
<body>
<div class="container-fluid">
<div class="row-fluid">
<div class="span12 red">Red area here.</div>
</div>
<div class="row-fluid">
<div class="span6 blue">Small box</div>
<div class="span6 blue">Small box</div>
</div>
</div>
</body>
</html>