Boring sky? Jazz it up in seconds using Photoshop Touch's Fade tool

Here's the file picker. You can only open one image at a time, so pick the biggest first, as the canvas for the project will be set to this size, and it's a pain to change it later

Tap the "plus" button down in the layers panel and add the second photo

Adjust the image to fit by dragging the handles. The magnet icon toggles "snap to edges" and the little chain link toggles the aspect ratio lock. Switch it off if you want to squash the image in any direction

Done. It doesn't come quite to the bottom of the layer below, but as we're losing this part of the picture, it doesn't matter

Tap the extras menu and tap "Add Fade"

Pick a fade style and move the handles to fine tune the transition. This looks pretty good already

Moving the handles closer together gives a sharper transition

Done. Pretty easy, right? But look closer and you'll see the clouds are in front of those towers

Here you see the problem (zoom in by pinching). Let's do something about that

Run your finger down the tools palette on the left side and find the eraser. Make it small and soft, and set the flow and opacity low so you can build up slowly. Erase those clouds, and remember: you can always undo

That's probably good enough, but those clouds, which came from an Instagram photo, are a little yellow. Tap the layer icon and pick "Match Color"

Adjust until you get what you're looking for. Photoshop changes the current layer to match the one below it

That looks about right. Tap "Apply" and you're done