I got this as a gift for Xmas and since it doesn't come with the DVD I didn't have any instructions. I knew exactly what it was, but it still took me more than a couple of minutes to figure out how to open it. If I didn't have both coins and that knowledge I doubt very much I would have "stumbled" onto the mechanism.
If you are really worried about it don't let one person hold both coins at the same time. But really, if your spectators are "examining" your coins it doesn't really matter if they can figure it out. Don't ask people to examine them...