Hello! I have to have a dual head stethoscope for school. I've tried searching online, but I don't understand the difference between a dual head and single head stethoscope. They look the same to me! Can anyone explain differences? Is there a difference in how they look? A family member has volunteered to buy mine as a gift, I want to make sure she understands what I need. I'd hate for her to spend money on something I couldn't use. Thanks in advance!