Hi Robert,
I put the pork butt on the top grate and the ribs on the bottom. My theory is the fats rendered from the butt will help tenderize the baby backs. I use 240 degrees as my target temp measured at the top rack. My baby backs were done in about 5 hours. The pork butt was about 7lbs and an estimate on the pork butt is 1.5 - 2 hours per pound. Normally for me a 7 lb butt usually takes me around 12 hours start to finish. However, this cook was unique and took 15 hours. This was the longest cook I have ever done. I don't know why this cook took so long. The weather conditions were ideal and no wind. The charcoal was fresh and not damp. I guess the meat itself was the reason why, but I don't know the reasoning. Possibly the breakdown of the connective tissues was the culprit.