When placing the two pork butts into the grill, make sure to leave a few inches of space between them. If they are too close together, they will cook like a single 18# pork roast. Since there will be more fat and connective tissue to render and more water vapor to evaporate, it will take more pellets to complete the cook of two butts vs a single one. Be sure to start with a full pellet hopper and check it from time to time during the cook so you do not run out. The overall cook time should not be significantly different whether you cook one or two butts.
If the cook seems to be taking too long, once you wrap the butts, you can always turn up the cook temperature to hurry things along. If you smoke the butt at 225F, you might turn the temperature up to 275F to finish the cook. Smoke only penetrates the meat during the unwrapped phase, so once you wrap it, it does not matter whether you cook it in the Traeger or in your kitchen oven.
Because you are cooking rather large pieces of meat, the cook temperature is important. Make sure you are checking the cook temperature with an 3rd party thermometer placed a few inches away from the meat. If you think the cook temperature is 225F based on the controller setting, but the actual temperature is 200F, this difference can add several hours to the duration of your cook, especially during the later stages of the cook.
