2 - 10 pounders will be o.k. but you are right on about not "overloading"...
The meat - being cool ( even if you allow it sit a while at room temp for before placing in the MES...which helps) will adsorb a lot of the BTUs being produced by the cooker - until the temp of the meat starts coming up. Then...we have the stall to consider. So, you could see perhaps an additional 2-4 hours total cook time to a potential target meat temp of 205F
205F is just a reference. I find that some butts will pull with little to no resistance at 10 degrees cooler. So, was for the bones protruding, and probe the meat with bamboo skewers or similar for that "like butter" consistency. Then you know it is done, regardless of temp.
Don't be afraid to push the cooking temp to 275F on these butts.