I just did this last weekend, and I have to echo one sentiment. Airflow became an issue for me. I was using an offset smoker a bit on the small side.
I wound up moving the butt much further away and they ended up getting done almost simultaneously, and on time. I was relived to say the least.
You might want to "preheat" a bit and get the temp up a bit past where you want to be for the cook. That way when you put the meat on, the drop isn't so bad.