Having worked/ volunteered at a couple railroad museums for the last 7 years, I can tell you that the more you learn about steam locomotives, the more you realize you don't know about them. Every single aspect of a steam engine's construction and operation is vastly more complex than most people, even railroaders outside of the steam environment, realize. The principles are simple, but there is much more than meets the eye. Don't feel bad about not understanding it all.
As Rich pointed out, power application is the key component to double heading. I was fireman on a steam double header last week. I was in the second locomotive and we were not working hardly at all. The lead engine did 90% of the pulling, we were just along for the ride. I still had to keep the fire hot, keep my steam pressure up, and have plenty of water in the boiler, but I wasn't working nearly as hard as my counterpart 70' ahead of me was.
