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.