Have you tried any of the suggestions previously mentioned? A few of the underlying messages I got from these guys was: 1) Socialization 2) Obedience classes ASAP.
If you plan to show in conformation or flyball, Rita needs to learn doggy manners. She needs to learn that there is absolutely no need for her act protective once she sees a strange dog approaching you. What kind of collar do you use? A prong might not be a bad idea. You will need to do major desensitization and heavy socialization. A quickie trip to a professional isn't going to fix all your problems =/
Also, it doesn't sound like you can necessarily handle both Rita and Sadie at once. I suggest walking them separately so that situations like the one aforementioned NEVER has to repeat itself. Additionally, do you tense up once Rita sees a strange dog? If you tense up, pull on the lead, and speak to her in angry tones, she is going to instinctively associate negative connotations with strange dogs. Have you any books on dog behavior problems? If not, I would recommend: How to Behave so Your Dog Behaves by Sophia Yin and in the case that Rita's aggression is fear based (sounds like it is to me), I would also suggest The Cautious Canine by Patricia McConnell.