In this case I would definitely go to someone else. Not because she works on two people at once but because 2 out of the 3 times you have been unhappy. If you are paying $125 get someone who does a good job. My stylist does another person while doing my color but I have never been unhappy with the job. Just like all professions there are some that are great at their job and some that just get by. It took me years to find a stylist that I was happy with.
I agree. The problem doesn't seem to be that she's doing other services while your hair is processing. The problem seems to be that you aren't happy with the service. I'd find someone else to do your hair so that you are happy with it. I once went to a hairdresser that a friend used (whose hair always looked beautiful) and had the same service that my friend usually had done and I was very unsatisfied with the job that she did. My hair looked terrible and the highlights were very chunky/noticable even though she did my friends highlights flawlessly. Needless to say I never returned to her and found someone else.