I have never seen any problem with hiring multiple therapists for whatever reason you want.
I never had a problem with breaks from either or both of the women. There were times I went without seeing either for a few weeks.
I understand seeing a consulting therapist, but other than that I don't get what the purpose of a back up therapist for you would be. What are you using the therapists for? Not that you have to tell me, but do you know?