well cant be too hard… two objects, two render layers, and use the black and white alpha mask think 
…you think well… …but it’s when it comes to the shadows and reflections that the fun begins!.. 
if you do it the way im thinking of, then the reflections should be fine. the shadows however… photoshop xD
no Photoshop, no Gimp… …straight from render!.. …not even cropped!
Pure entry
Blender cycles
