In PS Touch?
One way to do it would be to create a new project, then import each image. (The image you want on top would be, of course, on top in the layer stack.) Tap the topmost image in the layers to make sure it's selected.
Use any of the selection tools to select the area(s) you want to keep. Use the Edit icon (3rd icon from the upper left; looks like a pencil) > Extract. You can then tap outside the image to deselect the selection.