Cloning From Image To Image
If you're retouching an image using the Clone Stamp tool (S), not only can you clone from the image you're in but you can also clone from any other image that you have open. All you have to do is make sure both images are open at the same time. Go to the other image, Option-click (PC: Alt-click) on the area you want to clone from, switch back to the image you're working on, and then start painting. When you do, you'll be cloning image data from the other image.