Brian - I doubt if I am going to be much help but maybe some other folks will happen along. Sizing the brisket to the rack size gives you a couple of options. First solution is of course separating the flat from the point. Alternately, you could fold a portion of the flat under itself to fit the rack. Lastly, cut off only as much flat as required in order to fit the rack but the cut will bleed juices unless you can sear it shut/cauterize it in a very hot skillet. As for the butts, I recommend having a probe for each but if not, probe the smaller and when done place the probe in the larger. As for the brine, you can double it or simple flip the butts around from time to time. Are you planning on injecting your brisket with beef broth? Just a question.
Your start time is 19 or 20 hours prior to serving. Plenty of wriggle room as sometimes meat finishes sooner or later than planned. As each piece of meat reaches the desired internal temperature, double wrap in heavy foil, than wrap in a heavy bath towel, and place in a cooler. You'd be surprised how long the meat can stay very hot.
Lastly, once you have your smoker loaded up, relax.