One killer in a blond wig or two killers?

I missed the first half hour to hour of this movie when it ran on cable TV, so I have a question. Was Mrs. Johnson (Soledad Miranda) killing people in a blond wig, or were there two women killers? The woman who actually killed Dr. Huston and Dr. Johnson had short, coiffed blond hair, whereas the woman who lured Dr. Huston in the church and the woman who kidnapped and killed Dr. Donen had long, dark hair.

Two women killers acting in concert, or one woman killer who employed disguises?
And in either case, was it one actress or two?