We had a similar situation in that we had a male saint, and then added a female about a year after the male. Our male Henry, has always been pretty low key, he would bark but only once in a while. What we saw with the female, Darcy, was a far more protective attitude - which definitely startled us! We started to get really worried when she was about 5 months and we would go on walks and she would posture at some people who were just walking or jogging by. So we went back to socialization basics, having strangers give her treats, more trips to public places, etc. In addition to that, we reinforced our dominance in the house. She has to sit and wait until we say ok to eat, no treats until she does something for it, stuff like that.
It seems to have worked though. She is still a barker in the house when she hears an errant noise, and at the fence once in a while - when another dog walks by, but when we're walking or if they are in the car she has settled down. We've always had male saints and I have never experienced this aspect of behavior, I think females are maybe a little more protective!
