I wonder what we would discover if we put a cup of water in a 225°F pit and measured how long it would take to reach 212°F and then compared that to two separate cups of water sitting side by side reaching 212°F.

Would that have any relevance to this discussion?

Luckily, pellet grills do not deliver a fixed amount of energy like microwave ovens. When you add more cold mass, the grill needs to deliver more energy to maintain the set temp. It does this by running the auger longer and burning more pellets. Now, if you overload the system by adding more mass than the system can accommodate, set temp can't be maintained and it will take longer to cook. The other issue is that pellet grills cook primarily by convection. If you disrupt the airflow by overcrowding, you can seriously affect cooking times.

I'm not sure which model pellet grills are being discussed, but  unless you're talking about a small tailgate model, I can't imagine two pork butts causing either of these issues.

I wonder what we would discover if we put a cup of water in a 225°F pit and measured how long it would take to reach 212°F and then compared that to two separate cups of water sitting side by side reaching 212°F.

Would that have any relevance to this discussion?

My inclination is to think that if you have small mass and volume that you would not find much difference. That would be the case with the two cups of water. I think the issue may have more to do with the fact that the machine responds to temperature at the probe. Imagine this experiment. I have two cubes of meat. What happens as the cubes get larger, in fact, very large? The airspace gets smaller. The meat is an insulator so energy will be absorbed, but slowly. The outer warm layers of meat acts as an insulator to the cold core. The smoker only has to keep the air around the meat at the specified temperature. The smoker would therefore be producing less energy resulting in less entering the meat. This ignores energy loss, of which there is a lot. A smoker is not very efficient which is why they are hot to the touch. This is just one thought experiment. I am sure there are aspects of it that are correct, but I am also sure that it is far from complete.