People are different. Even if they do the same coursework and EC's, they are going to act differently, interview differently, and pick their nose differently. Speaking with identical twins, you can still tell them apart.
Some staff may notice they you two made an instant connection or feel like you are cliquey (you 2 might dominate a group interview - bad) - that may help or hurt your chances. On the plus side, having a friend there may help lower your stress levels, which could benefit you. That would also likely go out the window once you get to individual interviews.