<div id="box1">
<div id="left1"></div>
<div id="right1"></div>
</div>
#box1{100%;height:600px;position:relative;}
#left1{margin-right:200px;border:1px solid red;height:100%;}
#right1{200px;height:100%;position:absolute;top:0px;right:0px;border:1px solid blue;}