Not sure how much experience you have but since you're asking I will assume little so here it goes:
You have to remember you are servicing two girls (not the other way around). If you can get both girls in the mood and aroused then it always leads to a better experience. To paraphrase two other posters: oral, oral, oral to get everyone going. Then try both in missionary. Once the action really gets going, try one from behind while she goes down on the other, then, as the poster says, get the girls to 69 and fuck the girl on top. If the excitment level is there, the other girl will do unimaginable awesome things to her partner's vagina and your balls/cock. Pure heaven.
Have fun.