Chris-
Sure you can. I did four butts in my WSM this past weekend. You want to be careful that you don't have it so full that you restrict airflow and have temp control problems, but you should be able to get multiple butts and/or briskets on (whole briskets are big, so, you might be limited to two without having to get creative....I haven't cooked any briskets on my WSM yet.)
HTH,
Rich G.