stray voltage loop
by just looking at the wiring arrangement in the switch boxit appears to many as though the nuetral and the ground are the same thing but they are definitely not. the proper path for the electric is from the power copany and back to the power company which is theoretically the easiest path. the ground system is an alternate in case of some disruption purely for the sake of safety Now you want to add another box. It could be right next to the original or it could be in another part of the house away from the original.The code only allows you to ground from the original box where the power company feed comes into the house. you cannot put another ground wire into the gound from another location. So what you are doing is adding anothe buss bar to the nuetral and another to the ground buss.In effect it is really one box in two locations, with one grounding post. A loop takes too long to explain here, but that is why it is done that way. Also each black wire needs its own white wire to isolate each circuit. Don't try to feed two black wires with one white one, and don't cross over or "mixup" two circuits with different white wires. If you do you will never be certain that a tripped breaker has actually shut off a circuit.
Tim: Since I quit a bit early today, I'll wade in on this and hopefully try to make some sense out of it all. It hasn't been all that long ago that main panels had only one neutral/ground buss bar and it was tied directly to the neutral buss from the transformer which was also tied to the ground rod at the meter box. Sometimes the bare ground wire was carried on into the breaker box and sometimes it wasn't. The neutrals and grounds/bonds were mixed though out the length of the ground buss bar. There was nothing intrinsically unsafe with this arrangement, and thousands of homes are still being wired this way even today. What has happened is that in many cases systems have been added and a separate ground rod has been driven into the ground. Depending on soil conditions, or if grounded to a water line, metal properties, there can be a voltage potential difference between the different ground points. When these are tied together a current will flow between them, and the metal in one or the other will act as a sacrificial cathode and be eaten away. While the voltage difference may be small the currents between the two can be significant, and this current can become a source of electrical interference causing all kinds of problems with modern electronic devices. Many of the cases are worse case and would seldom happen is a real life situation, but in todays climate when nobody is responsible for their own actions codes need to protect them from themselves. Dale