In my opinion, if you are natural then you should hit each bodypart once a week. If you are on the juice then you can go for twice a week if you like.
However, if your routine is very basic(one or two exercises per body part, one for smaller, two for larger then twice a week is ok too.
Most people do not do 1 or 2 exercises per bodypart, so hit each bodypart only once, if they did twice, they would probably overtrain.
In the end of the day, it depends on your routine, genetics, and DRUGS