Photoshop: How do I keep 1x and 2x (for retina display) images in the same document?

I just read on an article about designing for iPhone retina display that you can keep the 2x images in the same document and use a Photoshop Action to scale up the 1x to see how the larger version will look. Other than creating a separate canvas, I’m not sure how this could be done? Would the action be designed to scale up the entire canvas, including all contents, to the larger pixel dimension?