Something like this?
In the CSS:
div.all { margin: 5px; width: 704px; height: 404px; border: 1px solid black; }
div.top { margin: 0px; width: 702px; height: 200px; border: 1px solid black; }
div.left { float: left; margin: 0px; width: 350px; height: 200px; border: 1px solid black; }
div.right { float: left; margin: 0px; width: 350px; height: 200px; border: 1px solid black; }
In the HTML body:
<div class="all">
<div class="top"></div>
<div class="left"></div>
<div class="right"></div>
</div>