Vance, Welcome Brother! It would help to know what you're cooking on. Most of us here list our gear in our signature line to help with such questions.
As a general rule I would say your usual allowance for time will be fine. A really full smoker presents a large thermal load and some additional time would be a good idea, but for 2 6# butts I'd say procede as normal.
Stop by Cattle Call and introduce yourself, and again Welcome.