You are absolutely correct. It is possible to have more than one disease/syndrome/disorder. MS is (thought to be) autoimmune and for some reason, once you have one autoimmune disorder, you are actually at risk for more. There seems to be some sort of predisposition to immune dysfuntion that can cause you to collect a host of problems.
I know one that has MS and lupus and I know another that has MS and diabetes. I guess I am lucky I just have MS.