Why is it important one is Best?
you dont need a "best" enjoy both their friendships :)
they could both be your best friend
friends are important and different friends give you a different perspective...it is important to keep both if that is possible....the best friend thing is something only you can feel at a heart or soul level and it really only need to exist there...you do not need to bring the label into the open.....It is like saying of two people that one has a disability and the other does not.....while we know the difference in ourselves it is not really important to identify it publicly
Don't be in such a rush to have one over another as a best friend. Friendship is something the evolves and develops as time passes. We humans while can interact to speed these friendships on a fast track, but all too often this will have undesirable results happening. Always better to let nature take it's time
There's no rule that says you can only have one best friend.