I italicized the 'two' because if you apply it to portraits it should read 'at least two'. Unless it's a self-portrait you'd have to include the person getting their portrait done.
"The Mona Lisa?" you ask skeptically, "Really? What does that have to do with photography?"
Digging into the cobwebs of my mind and dusting of the recollections of my Art History classes, the Mona Lisa was a 1500's painting that rocked the world for portraitures. She set a new standard. Before her time, 1300 and 1400 paintings were detailed, stiff, rigid, mostly profiles and full length. Mona, shall we call her, brought us close up, intimate, personal, in your face! Gone were the minute details of the background, things took on a haze, a blur, in photography, Leonardo opened up his aperture. The colors of the background and her clothing are muted not to distract from the subject, 'beautious' Mona. There has been an unending debate (the last I knew anyway) about the unmatching halves. Check them out. Is it a river, an upper lake, and how would they work together?
Now, check out her composition. She has been set in 'pyramid form'. Her hands are light and cross just off center, her elbow going out and up slightly. From there everything angles to the top of her head. This was a revelation in composition.