Diptychs got their start in ancient times when the literate wrote on hinged, waxed, wood tablets that could be folded to protect the words from nicks and marks. In the Middle Ages, those hinged pieces of wood protected sacred images rather than words. The two wooden panels became three and the triptych became the standard altar format.
Today any painting composed on more than one panel is called a diptych, triptych or polytych respectively. Together the panels should form one coherent composition, but they should also make sense (and be a good painting) if viewed separately.
Two Part Harmony is a stacked diptych born of necessity. I was painting at the beautiful Century Farm of the previous post and had no canvas tall enough for the scene I very much wanted to paint. But I did have two 8"x8" squares. So the rusty water tower and old-fashioned flowers could co-exist, just as they do in life.