There is so much medical information available (doubling every four years now) that it is not possible for any professional to have been exposed to all the information related to a particular symptom, diagnosis, or problem. In other words, one person can't know everything; each of us is limited by our training, our exposure to information and our experience with patients. The more serious the concerns, the more important it is to have more than one professional give their opinion. The most valuable second opinions come from experts in a very specific area, for example if you believe that your child is suffering from schizophrenia, look for a second opinion from someone who only works exclusively with patients who have schizophrenia.
I am happy to do an evaluation for the purpose of giving a second opinion, however keep in mind that I am not exclusively seeing only one type of patients. I consider myself to be well versed in ADHD, OCD, depressive disorders, most childhood disorders, schizophrenia, childhood autistic spectrum disorders, and issues requiring psychotherapy. Additionally, I bring a 3 year history as a pediatrician to my evaluation, in other words, I have a fairly good grasp on pediatric medical disorders.
In order to give a true second opinion, it is necessary to do a thorough evaluation, just as if I were giving a first opinion.