Will a single eeg pick up on more than one disorder?

Yes, but limited: Eeg is a very useful test in many different clinical situations, and can indeed provide clues for different diseases in the same patient. For example, a single eeg could show a focal area of irritation that is causing seizures, as well as a cardiac arrhythmia or superimposed metabolic problem. That being said, eeg findings should always be interpreted with the clinical setting in mind.