One method I commonly use is to wrap the left divs within another div for organization. You can implement this CSS:
#left{
float:left;
width: 45%;
}
#left1{
background-color:#ff0;
height: 100px;
}
#left2{
background-color:#f0f;
height: 100px;
}
#right{
background-color:#0ff;
width: 50%;
height: 200px;
float:right;
}
Here's an example of how to structure the HTML:
<div id="left">
<div id="left1">content</div>
<div id="left2">more content</div>
</div>
<div id="right">additional content</div>
Feel free to view a demo at http://jsfiddle.net/XnBe9/