I just saw this thread or I would have weighed-in earlier...
For use in print medium, you have no choice but to do a single-image overlay, but for online use you can do an animation. Animated overlays are IMO much more effective than single-image types, and require only limited local resources. You need to be able to flip one of the images and do an effective alignment between the two. You can do this manually by cropping so that the obverse/reverse images line up properly. Then upload the obverse and reverse images to one of the online animated gif makers, and create the animated overlay.
I can no longer link from Photobucket so you will need to look at my website to view these:
http://www.macrocoins.com/overlays.html
Contact me for photographic equipment or visit my home page at:
http://macrocoins.com