Why does my dog bark more than the rest?

OK, so I have two Rhodesian Ridgeback females. One I got from a breeder as a puppy who is now 1.5 years old, the other was a rescue that I got last March and is about 2.5 years old. My 1.5 year old girl doesn't bark much, only when she hears something that doesn't belong, otherwise, she doesn't even bark at the door bell or people we let in the house or even dogs we see on our walk. I have trained her so she leaves just about any person or animal alone when off leash. My other RR is a non-stop barker and I am struggling with this since I haven't really ever dealt with this behavior before. She barks at the neighbor kids, growls and barks at dogs we walk by, growls at people we walk by, barks at the door bell and growls at people until she knows they are OK. I left our door open with the screen door closed and someone rang the door bell and she literally launched herself at the door barking like mad, scared the poor woman to death, my other dog didn't bark but had her hair raised until I opened the door. How can I make my other girl stop freaking out? The rescue has no know info on her, she was simply found wondering around the city, skinny, sick, and looking desperately for something to eat.