One easy way...
On bottom image - Edit > Transform > Flip Vertical
Then, on same image - Layer > Layer Mask > Reveal ALL
Click on the layer mask
Set your foreground color to black
Go to your gradient tool
Set it to Linear Gradient
On the image mask, drag your cursor from the bottom
You can adjust the opacity to your preference and play around with it til you get what you want.
If you don't like my method, here are some links to some other variations.Easy Professional Reflections in Photoshop
I like to try setting the blending layers on the bottom image to either Multiply, Screen, or Overlay and messing with the opacity. It all depends on the colors, darkness, and background you want it to work on.
Hope this helps.