Not all fantasies are meant to come true. Some serve their purpose in creating arousal. Others need to be fulfilled or they linger and weigh on a person's mind. If you are to do it, all participants need to be fully aware of the expectations, their role in the play and all must be comfortable with their part.
If you go through with this, as an observer, you should not suddenly announce that you wish to join in. Is it to be a one on one meeting of two females with you as a voyeur or do you expect it to become a threesome?
you are such a giver!!