Cooking times info for a green horn.

I know this is going to be a silly question but I am truly a newbie to grilling and smoking. I have done some things on BGE but have only had my YS640 a little over a week. I know the general rule of thumb is 1 hour per pound on a boston butt. If I place two butts on the smoker (bottom shelf) will the cook time be about the same as for one or will it increase? If so about how much? Any advise, suggestions or just plain old making fun of the new guy is welcome. Thanks