My 2 cents on this...
2 girls do not always feel comfortable with each other in the same room.
For ex, if you have a therapist you like and gives great service and you want to add her friend along this next time (but the friend might not give as great of service) that may set the limit for the whole session. They try to keep the same level of service as the other so that usually means a lower lever. Hope I'm making sense.
So unless you get the right 2, then just keep getting the 1 girl.
Usually you can get your main girl to reccommend who she would work well with vs you just picking the 2nd girl.