Assumimg you don't know this date much yet, as you already know; she is preventing isolation by doing this and also not intrested much in you becasue she wants someone else to talk to or she is shy, don't be upset, if the friend is attractive by your eyes try to get her contact and then see if she is intrested in a seperate date sometime.
If they both ignore you; cut it short. If they are includeing you in the conversation you can alternate attention between them and see if one of them works to get more of your attention. It can be alot of fun if you get them competeing for your attention (durring the time) but if they sort of ignore you or move the conversation into topics that don't make you feel desired or are things and people that you don't know excuse yourself and exit right away.