I love to challenge myself to paint a Diptych every now and again. Its most important that the each painting has a strong composition as well as reading as part of a pair. That pair must read harmoniously and together have an equally strong impact.
Its also for esthetic reasons as in my home the contractor decided that it was a good idea to put the thermostat right in the middle of a hallway. As a result I cant hang one big painting but a diptych works perfectly.
Two paintings work well in a master bedroom or on either side of a statement piece of furniture.