While they both would most likely fit, I personally would do them on different kettles. I tend to do my pork butts at a higher temp (275) and my Briskets lower (225ish). Just my preference as the butts are marbled enough that they can take the higher heat no problem and the briskets (at least the ones I can find) are not so much so I prefer the lower and slower with those. Having the both together would also double the time you had the lid off to check and most likely slow the cooking process down some, but again, that is just what I have run into during my cooks. I am sure others have totally different experiences, but brisket at hot and fast has never worked well for me. I would do a 2x2 snake and start with 12 chucks in the chimney.