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I would be interested in an update since it's been about two weeks since you originally posted.
I have known several people in open relationships and attended a swingers group that met at a bar where I used to live. The couples there had been at it for awhile and were very comfortable sharing, but they almost all had the same rule: start the night together, end the night together. Everyone of the couples knew that their partner was there with them regardless of who they were currently having sex with, and they always came back together at the end of the night.
I mention this because it would be my advice to you : if girl 1 is not comfortable with girl 2, than forget girl 2 because another girl 2 will come around eventually. until she does, be with girl 1, go to the swingers clubs, and everything under her conditions, and slowly open her up to the ideas of being in an open relationship remembering the rule above. If you always make sure she knows her place in your life and the relationship, she will either be comfortable enough to let you have girl 2 (or whoever girl 2 is at the moment) or she will stay the same. If she proves to stay the same, than you might want to reevaluate your feelings about open relationships, or find someone who's really interested in one.