Well for me in my first one I did the one girl doggy style while she ate out the other who was laying on a bed. Then I had one with 3 girls and me that was a bit confusing, but at one point two of them were blowing me at the same time, I was banging two of them at the same time, etc. Only problem with that is one person is pretty much left out at all times.
Then this one time uh.......the one girl laid on the bed, with the other girl standing at the end of it eating her out, and then I was kneeling on the other end of the bed with the first girl giving me head. And then this one time was in a hot tub and like I was sitting down with one girl on top of me, and then the other girl was like switching between playing with my bawls/fondling the other girl's clit, and making out with the other chick, etc.
IMO the best way is to let the two girls go at it for a while and just watch. Then when you get bored of that just sort of wait for an ass to end up in the air or another oppurtunity to get yourself in there, and jump right in.