Totally agree with Mike... you can do Mac & Cheese in 2 hours or less which is a good rest time for the butt.
If you do them at the same time you're going to want to put the mac and cheese above the pork so it doesn't get slopped. You're also going want to put it in at the beginning so that you have some good smoke. This means two things: You're going to open the smoker while the butt is cooking letting out heat, moisture, and smoke. You're going to have mac and cheese getting cold for 10 hours.