Find a way to get both of them in the same place at the same time. If they both think you're cute or whatever then the hard work is done. Take them to a club, bar, whatever. A little liquor and their inhibitions are removed.
Flirt with both of them equally through the night. If nothing else, jealousy will get you at least one of them. Make sure you have a comfortable place for both of them to go with you at the end of the night. Your house/apartment whatever. Make sure no dumbass roomates will be around to heckle you or try to steal one of them. That can make them uncomfortable.
From there, just put something on the TV, sit on the couch with your arms around both of them, and slowly work yourself to kissing them both. Keep an attitude of "You're both lucky to be hanging out with me, not the other way around" and they'll pick up on that confidence.
Just don't get more drunk than them. I've failed many times because I was more drunk than the girl I was trying to get with. You have to appear witty and smarter than the girl you're with.
