SO custom responsive row

HTML

<script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.1/js/bootstrap.min.js"></script>
<div class="container-fluid">
    <div class="row-fluid">
    <div id="myspan1" class="span12">span12</div>
    <div id="myspan2" class="span12"12></div>
         <div id="myspan2" class="span6">6</div>
         <div id="myspan2" class="span6">6</div>
            <div id="myspan2" class="span4">4</div>
            <div id="myspan2" class="span4">4</div>
            <div id="myspan2" class="span4">4</div>
</div>

<div class="row-fluid">
    <div id="myspan1" style="display:none;"></div>
    <div id="myspan2" class="span12">span12</div>
</div>

<div class="row-fluid">
    <div id="myspan1" class="span6">span6</div>
    <div id="myspan2" class="span6">span6</div>
</div>
    
</div><!-- end container -->

CSS

@import url("http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.1/css/bootstrap-combined.min.css");

.row-fluid{
    height:100px;
    margin-bottom:10px;
}

.span12, .span6{
    background-color:#aaa;
}

#myspan2.span12{
    margin-left: 0;
}