You can open the second image, then press CTRL+A to select all, then hold down V (the move tool ) and drag up to the tab of the first image. Usually you have to hold it there a second before PS switches tabs. I believe if you hold the Shift key down while moving the layer around it will automatically line the layer up (if its duplicate images, say for a split ND filter effect with 2 different exposures ). The reason why I show you this way, is because when you are cutting out portions of the second image, its easy to just press "V" and drag. You can do the same once you place the layer, if you want to re-situate it. Just make sure you have the correct layer selected or you will start messing up other layers.