I think it is worth mentioning that although Silkstone has used two images of the same shot, you can use different images. If you wanted to replace the sky, for instance.
If this is what you want to do, you might need to ensure that the two images are the same size and dots per inch etc (not always necessary, but probably easier to work with at first), and you might need to select the sky you want to remove and, well, remove it from that image before you blend the two together.
These two images have the same sky - and that was shot from my bedroom window, not in York!
Is this what you are trying to achieve?