In photography terms, a Diptych is essentially the placing of 2 photographs next to each other to present a form of storytelling with the images. There’s no rule that the photographs have to be related but it certainly helps when they are so as to maintain some connectivity between them.
I often create diptychs by using a Photoshop template that I made. The process of manually resizing each photograph to specific dimensions that I require for any image used on my site is painful at times but I’m unaware of another way to accomplish this until I stumbled upon Storyboard for Photoshop. In the words of the creators:
Storyboard is a Photoshop script designed for photographers who enjoy displaying their work in dynamic and fluid blog posts. It makes laying out images on your blog amazingly quick and unbelievably easy. No more custom templates and no more resizing images by hand.
The plugin runs for $54.99 and what sells me most about it is how fast and configurable the whole process is which you can read more about.
If composing Diptychs is something you still want to do but you don’t want to dish out the $54.99 for the plugin because you don’t do it enough to justify the purchase, then you might want to consider this great tutorial by Jay Watson where he uses Lightroom instead.