.contenedor { overflow: auto; }

.div1 { float:left; width:440px;}
.div2 { float:right; width:440px;}
	
#header {
    height: 100px;
    background: #0072b8;
    font-family: Arial;
    font-size: 16px;
    color: white;

}
	
<div id="header">
        <a href="home.html"><img src="http://placekitten.com/237/100" width="237px" height="100px" border="0" style="padding: 0 3.5em; float: left;"></a>
    </div>
	
<div class="contenedor">
    <div class="div1">
      <a href="http://placekitten.com/300/305" class="lightbox"><img src="http://placekitten.com/300/305" width="300px" height="305px"/></a>         
    </div>
    <div class="div2">
       <a href="http://placekitten.com/300/305" class="lightbox"><img src="http://placekitten.com/300/305" width="300px" height="305px"/></a>
    </div>
</div>
	
html {
overflow-y:scroll;
}