If I got your question correctly,
Your CSS should look like
#container {width:100%; height:auto; overflow: hidden;}
#content-left {width:50%; float:left}
#content-right {width:50%; float:left}
And HTML markup
<div id="container">
<div id="content-left"></div>
<div id="content-right"></div>
</div>
The full code
<html>
<head>
<style>
#container {width:100%; height:auto}
#content-left {width:50%; float:left; height:50px; background-color:black}
#content-right {width:50%; float:left; height:50px; background-color:red}
</style>
</head>
<div id="container">
<div id="content-left"></div>
<div id="content-right"></div>
</div>
</html>
*note: I've added height:50px; background-color:black to see the result. You can remove this code, of course
And here is the example
http://jsfiddle.net/QbLgu/
This is the basic things that every webdev MUST KNOW. You need to learn basics. Try to research, before asking question on SO