Do I need to buy them birthday presents?

Two of my friends invited me and some other girls to go to a concert for their birthdays (they are not twins), and one is one of my best friends. The other girl and I are friendly but not overly close. I am paying 80$ to go to the concert and wasn't sure if I need to buy them both a present. (My first thought is only get one for my best friend) Your thoughts and reasoning behind your answers would be very helpful, thank you!