I was in this same situation myself and there really is no easy way out. This is a bad idea altogether. If you are going to hurt either one of these girls they will both hate you because they are best friends. Being in this sort of situation myself I won't be able to give you an answer to this exact question but I will be able to give you a solution to your problem. In my particular case I decided to stay a friend to both girls without favoring either relationship and the three of us are the best of friends. I get affection from both of them and nobody gets jealous or hurt. I am very happy with the relationship I have with these girls and I wouldn't have it any other way. If you really are intent on leaving one girl for another then I suggest not actually telling the girl anything but as nicely and as slowly as possible show the girl you want to be with more affection than the other and eventually she will see what is going on. A relationship with two girls could be related to an old fashioned scale. It can be balanced, or each side will be complete opposites to each other. One side may go up but keep in mind the other side will always go down correspondingly. I hope this helped and good luck, you'll need it.
Sometimes many of people also got in situation like that. As for me, you should pick one of them instead of choosing both of them...you should keep improving with one of them, must've one of his/her friend will understand your situation (I hope so) and let you with his/her friend to become in a relationship but make sure that your girl/boy still have time for their besties :)