Two 10# butts would be pretty crowded in the #1, but it's possible if you can somehow arrange them with some space in between. Depends on the shape. You can always smoke the first butt for 6 or so hours, then wrap it in foil or into a pan tightly covered with foil, with some apple juice or water, then finish in a 225-250 oven to 195-205 internal temp, while you smoke the second butt. Once butts get to about 150-160 internal temp, it has plenty of smoke, and you can finish it in the oven. Your first butt will be done before the second, but you can hold it or reheat it if you need both at the same time.