It is quite likely that this is a legit situation with two women traveling together but not entirely being together the whole time. It makes sense that one may arrive and stay one night and then their friend or colleague meet up with them before going on with their trip. Just ask why they are doing it this way and get a feel for the situation.
If the booking guest is communicative and polite I would hope this is going to work out. Especially since both guests are women. Ask for the name and city/origin of the arriving guest and their phone number. Call the arriving guest and confirm her name. Chat with her about her planned arrival. Get a feel for her personality and demeanor. Be reassured.
If the booking guest feels uncomfortable about providing the arriving guest's information to you (after all you too are a stranger to them) tell the booking guest that she must have the arriving guest call you immediately so that the guest herself can give you the necessary information.
If neither peson will reveal the arriving guest's name, or if the arriving guest will not talk to you by phone or email before their arrival then call Airbnb and insist on a cancellation because the guests are not willing to let you know who will be arriving or allow you to communicate with that person about their access to your home.