I know someone who completed his bachelor in Chemical Eng..., had OPT for  a year, went back to college to join Medical Terchnology degree and had OPT again for second time at the same level of study. I am not sure exactly what is the creteria for this. My assumption was same like many of yours -- one OPT at the same level of degree. Just to make sure I called this person and he confirmed me that he got OPT for second time and waiting for his H1...........Another thing when you use full year of full time CPT , you don't qualify for OPT for that study or degree. However, you can  get permission not to enroll in classes when you have fulltime CPT. I am not sure about the time period either it is whole year or 1 semster...............

thanks...............