I think of having 6 zones of heat with this setup. Zone 5, right over the heat, is generally where you want to cook steaks, chops, burgers, etc. Hot dogs, sausages, etc. I usually put in Zone 2, which is on the top grate right above the coals. Zones 1, 3, 4 and 6 are where I put the meat that is to be slow cooked. So pork shoulders, whole roast or jerked chicken, chuck roasts, etc. go in those zones.
If I am slow cooking, I don't ever need the area right over the heat source, so I take that grate out to make adding fuel to the fire easier. When it's time to add more wood or coals, I can pop the top open, add more fuel and close it quickly without losing too much heat. The more you have to fiddle around with stuff, the more heat is escaping, the more you'll need to wait because it will take that much longer to get back up to temperature. That's one of the reasons why most BBQ smokers have a separate box to add fuel to. You can manage the fuel without letting valuable heat escape. No matter what your setup you want to make it as easy as possible to add fuel during cooking to minimize heat loss.
Besides minimizing heat loss, another oft overlooked element of heat to cook with for BBQ by neophytes is thermal mass. You can find all kinds of fancy definitions of this online if you wish. The parts that matter for us are how much food you have to cook and how your smoker or grill is at holding heat. Something that has more thermal mass, say a brick oven, will hold a steadier heat for a longer time and at a more constant rate than a thin piece of aluminum. Also, if you put in one pork shoulder put on the smoker will soak up a lot less initial energy than if you put 10 on at the same time. This is one of the reason cheaply made grills end up being lousy for bbq. they are flimsy and made of materials that end up not holding a lot of heat. Thin walls, a lid that doesn't close well and flimsy grates will be less effective than thicker walls, a snug fitting lid and thick cast iron grates. Adding various things like firebricks, baking stones or hunks of metal depending on your desire and budget are also options to consider if you start taking this stuff seriously.
Incidentally this is another reason a firebox makes sense. With a separate firebox that is loaded up with fuel and good air control, you can load the box up with fuel and have a good chance of having a big pile of fuel vs. the small pile of fuel I have in my set up. If you have the meat a couple of feet or more from the heat source vs. 6 to 18 inches, you can make the fire hotter and larger, because the heat will dissipate a bit before it gets up to the food. The larger mass will, if you control the air flow properly, burn more slowly, thereby requiring you to add fuel less often.
Whenever people talk about cooking BBQ they talk about low and slow, cooking over low heat for a long period of time. Whatever your setup, take few minutes to think about how you can minimize heat loss and increase thermal mass. While you may not have the money, time, room or desire to build your own double barrel smoker, taking some time to consider what makes them good at their job and applying those principles as best you can to your current rig can have a tremendous effect on the BBQ you produce.
