They will only stay hot a few hours in a cooler. To a small extent they will cook a bit more, but mostly the juices will redistribute themselves inside the roasts.
What I suggest is to smoke the butts the day before, then let them cool completely in pans and then foil the pans and refrigerate. The next day, warm them in a 300F oven for an hour and they will be piping hot and ready to pull and eat. I've done this several times for large cooks, where I needed the smoker one day to smoke the chuck roasts and pork butts and the next to do the ribs.