TD,
Glad to see you posting here and THANK YOU for choosing the same name so I know who you are! Lots of good advice above, but if you've spent time here, you already know that.
Doubling the load will not double the time, but it will extend it. When we prepare for C&B, I do (4) 6-9# butts at a time and it takes about 14-18 hours depending on total mass. When we do the other stuff on-site, we cram about 35-40# per smoker and most of it is done within 6 hours. The difference is the on-site stuff is smaller & not a big hunk of love.
Also especially important when dealing with the larger cuts is to take them out of the fridge an hour or so before placing in the smoker. A meat mass at 40F will take its' toll on the smoker temp a lot longer than one at 65F.
Kirk