As a postscript, these turned out pretty well. My thanks to those who replied.
My only concern was when the slabs plateau'd at hour 12, and stayed at 139 degrees until hour 15 when serving time was rapidly approaching. I was shooting for an internal temperature of 200-205. After finally rising to about 170, I did what I'm told is called a "Texas cheat", and wrapped them tightly in foil in a 300 degree oven for the last couple hours. That seemed to do the trick.
We served the larger one at dinner at a friends house and it was well received. Gave half of the second one to our hosts, and I had planned on some bbq sandwiches last night. I brought home some hard rolls last evening to that effect, only to find that my family had beaten me to the brisket. There was barely enough left for 2 sandwiches.

I guess I can take that as a complement, though.

P.S. I saw the whole briskets at the Business Costco again last night.

I'll try one of those when I have a couple days to devote to it.