Well, are either of these girls your girlfriend or a girl that you have feelings for? Do either of them have feelings for you?
If the answer is yes to either of those questions, don't do it, dude, because a lot of drama and jealousy is about to ensue that will make the cost much higher than the benefit of the coveted threesome.
If you are all seeking JUST sexual pleasure, GO FOR IT! Have an awesome time, but please, for the love of god, use a condom. The last thing you want is TWO babies or any number of STD's (not saying that these girls are promiscuous or unsafe, but you never know). Good luck!