Eric, something is amiss. Two 8 pound butts should never take 20 hours at 250-275. My immediate thought is that you had a bad dome thermometer reading. I once had a 4 pound mini-butt to cook for 10 hours and still never reached 190 and then the next day I happened to look at my dome thermometer and saw it registering 140 - with no fire going. I then realized I had been cooking that butt at about 190. No wonder it didn't do better.