Explain why, please. I'm not an electrician, but as I understand it, when you install an electrical socket the same three wires in the romex cable provide power for both plugs. Effectively each of the holes in the top plug is bonded to the same hole in the bottom plug. So why would connecting them with an external wire cause a problem? Even if two outlets are right next to each other, and one has reversed polarity, you'd just be connecting neutral to neutral and hot to hot. I don't understand why it's a fire hazard.

edit: last two sentences are wrong. If one plug has reversed polarity and one doesn't you'd be connecting two wires from hot to neutral, and that would probably just blow a fuse.