If i have 2 boston butts and want to cook them in the same smoker....?

No, you will not need much of an increase in time. The smoker will take longer to come up to temperature but once it hits that 200f - 220f mark, it should stay there regardless of the amount of meat on the rack. My guess is that it will take 1/2 hour to 45 minutes more to get to temp.

The way to not worry about the time is to use Temperature. Put in a remote thermometer in one of them and when the temp reaches from 160F it will be done. Keep in mind it will continue to cook after you remove it from the smoker and to let it rest for at least 30 min. It's best to cook low and slow as it will be more tender and juicy. The smoker should not get much hotter than say 225F or maybe a little higher but not over 250F. Time is relative to the size of the smoker and the temperature of the meat along with size and starting temperature if the meat sot time is not a good way to gauge it.

Oh almost forgot make sure there is plenty of water in the pan or that you use soaked seasoned wood. Dry wood and no water will make for dry meat.

No; you can cook both for the same amount of time there is only 1# difference