Based on a picnic shoulder I did last week, I would check the temp around 10 hours to see how close you will be, 16-20 hours seems a little long. I did a 9.5# shoulder and pulled it from the smoker at 205 degrees. It only took 12 hours. Your butts are 8 and 9 lbs, I would think it they would cook in about the same amount of time as mine. I tried to maintain a pit temp of 225 and bumped it up a bit higher for the last 2 hours.
Gator Pit BYCC
Weber 1100 Gas with Cast Iron Grates
Kenmore Kettle Charcoal grill