I use the bell method as well 
I don't know about your system, but I know with mine I wouldn't be able to hear it from the distance you're describing. My dogs whine/bark a lot louder than they ring the bell. If she's whining to get back in I don't see a reason to put another bell outside. It's possible that it could confuse her as well, since ring the bell = go outside to potty.
I don't know about your system, but I know with mine I wouldn't be able to hear it from the distance you're describing. My dogs whine/bark a lot louder than they ring the bell. If she's whining to get back in I don't see a reason to put another bell outside. It's possible that it could confuse her as well, since ring the bell = go outside to potty.