Are you referring to the vertical bar of color running down the middle of the image? If you are, you read Help:Theme Designer. Make sure the "no split" option is unchecked. If you are referring to the image itself, that is just how the image is; it isn't two different images. You can see their background image here. They use custom CSS to add it but they could have just added it the normal way to. I am not sure why they used CSS to do it.