4 LB time wise. And yes figure about 1.5 hrs per LB. If it gets gone early wrap in foil and put in a cooler with towels. It will stay hot for hours. Remember to post a Qview.
You are cooking 2 -4 pound pieces. The cooker doesn't know or care how many. You always cook according to the size of a single piece. These guys that Q fifty at a time would be standing there for a month if the cooking time was cumulative.
As said above, 2 Butts, 4 pounds each won't take as long as an 8 pounder.
If it's in a tightly enclosed smoker like an MES, it would take slightly longer than only one 4 pound Butt, because you're starting with twice as much cold meat adding cold to the inside of the smoker for awhile, however that shouldn't add any more than 15 or 20 minutes to the whole smoke time.
A rough estimate would still be about 1.5 hours per pound @ 230° smoker temp, and foiling at about 165° IT.
All 10# and cooked as one ... it was 18hrs. counting stall , but the more you fill your Smoker the more efficient it will be. That's using one type and similar weights , different types of
meat with different times to cook , will take longer as you keep opening the lid and letting all the 'love' out...
Your secret to cooking large hunks of meat is to 'leave the dang lid/door shut' , these had no intervention at all and look at the Bark , like Mahogany...and they all fell apart when I tried to
I will correct myself , opening the Smoker with anything in there will only delay finish times and usually you get a darker color.
Good cooking practices and Smoke production , will reward you with a beautiful meal...
Happy Smoking , a Merry Christmas and a Happy ,Happy , Happy Nuevo Anos ...
Have fun and . . .