You can also do this within the one image by duplicating the background layer onto a new layer. Select the top layer, then go to Edit > Transform > Flip Horizontal/Vertical. Although similar to Canvas Flip, it doesn't flip every layer, just the selected one.
At this point, you would probably need to make the canvas a little bigger (Image > Canvas Size), then create a layer mask for the top layer. Move it around to suit, then paint onto the layer mask with a soft brush or gradient to make the two images blend correctly.