1.
Have you considered using a background image in the outer container to achieve the desired effect? This background could be the desired width and only 1px in height, featuring two colors side by side and repeated vertically.
2.
Another option is to utilize CSS3 with a horizontal gradient. This method offers more flexibility as it does not rely on an image, but rather on a CSS3 gradient. You can see an example of this approach here: http://jsfiddle.net/VBzau/13/
The gradient used can be created using tools like
3.
Alternatively, you can use masking techniques by incorporating images that act as negatives of border radius and match the background color where the box is positioned.
An example of this technique in action can be found as a fallback in the main content area of this website:
4.
Considering your comments on structure, I would recommend the following adjustments. Take a look at this implementation: http://jsfiddle.net/VBzau/14/
In this suggestion, I have added another container around the main container, re-applied overflow:hidden;, removed the corner radii from the left box, and relocated the image within the new outer-container.