Just because YOU know them both, doesn't mean that they know/like each other, and will work together nicely.  If it were me, I'd decide which of the 2 was more important to me, and mention to her that I'd like to see her & a friend.  I'd then mention the other girl as a possibility, and ask if she'd be comfortable with her, or had another suggestion.

Good luck & have fun.

If *you've* met both ladies before, and they haven't yet met each other (that's the impression I got from your post), introduce them and let them mingle a little bit before you jump in and rip their clothes off. They both already know you, so they just have to figure out how to gauge each others reactions and levels of attraction before they jump on each other. Try to rush it, and you'll do nothing put but the breaks on the whole shebang.

That also said, don't be greedy with the ladies. There's a lot of movement in group activities such as these, and we tend to feel restricted if you've pinned us down for a lay while not leaving enough space for your lovely girlfriend to sit on our faces at the same time. That's just bad manners! Just be aware of what's being done to who, and since you already know the ladies involved, you shouldn't have difficulty reading their body language and making sure they get what they want

Fun times!

Maxine