Here is another example of merging two photos together using blend modes.
I took a photo of a group of jets flying mid air with a clear sky and merged a photo of a cloudy sky together to get this composition. I used the Overlay blend mode, because I wanted to increase the color of the sky and accentuate the smoke from the jets. Overlay got the job done.
*tip: the easiest compositions have little to no distracting elements when used to merge with another photo. For example, having a clear sky, makes it easier to blend in clouds or birds or other elements that would take up the space of the sky.