The connection may be a place, a theme, a colour, the same subject taken at different angles or perspectives, a shift of focus, cause and effect, same subject -different time of the day, etc etc - the possibilities are endless. 

I love a diptych because it forces you to think about your images a little differently, and often makes you switch from micro to macro view, from person to environment, from action to outcome. 

If you're shooting with a diptych in mind, you shoot more diverse images and your photography gets all the better for it. With a diptych you pay attention to storytelling, you pause on the details that you may have otherwise left uncaptured. But equally, you pay attention to the wider scene and how your subject might interact with it. 

Now, I said we will be doing a Spring Diptych, which means you need to find a way of incorporating some visual clues that hint at spring in at least one of the image. Perhaps you want to include blossom on the trees or a carpet of flowers in the big picture? Or maybe a single flower or a spring lamb in another? Think about what you're shooting and how it might complement or contrast with another shot.