:nth-child for span6 bootstrap

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css">
<script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/js/bootstrap.min.js"></script>
<div class="row-fluid">
    <div class="span6">1<br>sadsada</div>
    <div class="span6">2</div>
    <div class="span6">3</div>
    <div class="span6">4</div>
    <div class="span6">5</div>
    <div class="span6">6</div>
    <div class="span6">7</div>
    <div class="span6">8</div>
    <div class="span6">9</div>
    <div class="span6">10</div>
</div>

CSS

div.row-fluid div.span6 {
  background: #ccc;
  border-radius: 8px;
  margin-bottom: 10px;
  text-align: center;
}
div.row-fluid div.span6:nth-child(2n+3) {  
  margin-left: 0;
  float:left;
}