I first learned the word Diptych while in art school. I was asked to critique a fellow grad student's work, which consisted of 2 square wood panels, about 16"x 16" and 2" deep, was layered in skin-toned wax and embedded with horse hair, the tufts of which had been cut at different lengths. "It's a diptych" the artist added, meaning to be helpful.
While I never further forayed into Diptychs while in art school, I have found myself experimenting with them in the presentation of my photos. At first, because my website is best designed to feature landscape oriented pictures - I felt like there was a lot of space on either side of my portrait oriented pics. So I started putting 2 images together, like this above.
A diptych, in an artistic sense, is a fancy name for an artwork consisting of 2 pieces. They work together. That's it. So when I put photos together, I try to choose ones that show different aspects of a person. I could do Smiling and Crying. Here we have Close Up and Far Away. The images could contrast, or they could match. The photos are placed so that they function compositionally as one. For instance, had I placed the tiptoe photo on the left of the face photo, the tiptoe photo would point the viewer's eye out of the frame of the pic, instead of towards the face, making for an entirely different composition.