To be fair, it depends upon the personal statement.
The usual topic of a personal statement is "Why I want to do XXX". Personally, I think this is rather boring because your reasons are not much different than anyone else's. In Internal Medicine, there really are only 4 reasons ever mentioned: 1) I like solving puzzles; 2) I have/had this disease and I want to treat others with it; 3) Someone else has/had this disease, and I want to treat others with it; 4) I like talking to / connecting with patients. The fifth type of personal statement is a rehash of your CV in prose format. Those 5 PS's cover 95+% of all PS's I see. In these cases, you'll usually need two PS's.
If you actually write an interesting personal statement, perhaps actually telling me something about you, that's much better IMHO. In that case, only a single PS might work for two different fields. It depends upon the fields and how similar they are, for example.