Diptychs are 2 photos displayed together that combine to show an idea that is often bigger than either photo standing on its own can do. There are tons of ways to do this. It can be done side by side, top and bottom, obvious connection, subtle connection........whatever connection the photographer wishes to make. I love looking at them.
Just Google Diptych Photography and see what you find. Here's a simple one I made...mostly because I spotted the scene while driving 60 mph, and by the time I did a U turn to get the shot - it was too dangerous to get out of my car and get the whole wide angle.....
