I use the trimmed weight for my estimate .. but it is only a rough estimate. I rely solely on the internal temperature and probing for tenderness to determine when it is done.
The time required to smoke multiple briskets is generally the same as smoking a single. If there is a big difference in size (weight) I place the larger one on a rack below the smaller one. If you only have one probe I would insert it into the smaller size but I have found there is generally little difference on them reaching the proper temperature. I did have one instance when one brisket was tender a couple hours before the second but generally they are done within a half hour of each other.
I change the foil in my smoker after each smoke. The bottom will still get a greasy film on it but you can just wipe that clean again. I do not wash the door or sides but will brush off any flaky deposits that may form.
You will learn something from each smoke you do. It helps to keep notes from each smoke to make any changes you might try to determine what works best for you. I have my best results smoking at 225 while a buddy of mine .. using a different smoker .. smokes at 260. After a while you might end up being the family restaurant. I often get requests to provide the meat for family gatherings either here at home or elsewhere. Try double smoking a ham sometime. It is another family favorite.
