A 6 pound butt and a 9 pound packer should not be a problem on two racks. I think the overall setup of meat and racks makes sense. Heck, I have done same on my 18" kettle. Running at 250F, it will run through the night, I would bet 12 hours or more. I would put the probe into the fattest part of the flat, that is going to be the part that you need to monitor the most. That being said, internal temperature only matters as to when to start probing. If the butt has a bone in it, cook it until the bone wiggles freely. If no bone, then cook it until you can reach in and easily split the butt in half.
The brisket should be probed at 190F internal temperature, if it is really tight, let it run for an hour. Then probe again. Do this until it probes with the chosen device sliding in easily. An ice pick is the ideal profile for this job. Since you will be stacking, use a long skewer, that works just fine. And reduces the risk of dropping the butt as you try to probe the brisket.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."