In this case, it's a little odd for the person in the foreground to look smaller than the person she's in front of. Perspective would dictate that it be the other way around.

You might also experiment with making the two people more similar colors (edit the color levels for each person before pasting), or applying textures, color tweaks, and filters to the whole picture after the pasting. Basically, make it look like they're in the same color of light.