The temperatures have been relatively mild lately so this might not be a useful observation but, with both stoves running at low settings, I am getting plenty of heat, longer burns, and more thorough burns. The ash is mostly a fine powder with very few chunks and even fewer pieces of unburned coal. I'm sure that I'm burning as much coal as before however, the house is considerably warmer.
I don't always look forward to shaking down two stoves and dumping two ashpans. When the weather gets really cold, I won't mind it so much.