OK, everyone: I'm going to do this in two stages, and this is the first one. I'll post photos later, in a few minutes.
I encountered every problem I've been reading about, and I used bits and pieces of everyone's advice with this cook.
BTW, the pork butts were DEVOURED! More on my experiment later.
Everything was rolling along smoothly until the temp hit in the 170's. There was no stall, but the rise in temp slowed considerably. When the internal temp hit 180, it was as though I entered a barbecue twilight zone. At 183, about an hour later, I don't know what happened. The temp stayed there for almost three hours, falling back to 181 in between. I gave up watching the ET-732, and went for ice. When I got back, about a half hour later, the temp was at 190. It stayed there for about 45 minutes when it hit 194. It hung at 194 for maybe fifteen minutes or so, then jumped to 196, which is when I took the meat off the smoker. When I went to take the butts off the smoker, they were practically falling apart as I swung from smoker to table, wrapped both in foil, towels and in the cooler. Guests arrived promptly at 5pm, whereupon I removed the butts from the foil, pulled them, and I broke the ice by making the first sandwich.
My experiment -- I used Myron Mixon's marinade: 3 cups apple juice, 1 cup white vinegar, 3/4 cup sugar, 3/4 cup kosher salt. For the second butt I varied the marinade slightly: 3 cups apple cider, 1 cup apple cider vinegar, 1/2 cup sugar, 1/2 cup kosher salt. When I removed the butts from the smoker, I forgot to note which was which. I figured it out later because the original marinade recipe tasted a bit saltier to me. The butt with my version of the marinade went first; there's still some of the other left over.
I put the butts on the smoker at 4:55am, and they hit temp at 4:05pm. When I was checking the temp every hour, using the Taylor pocket digital, I was getting different readings from different parts of each butt. So, I just averaged the readings. After a while, when I ran into problems with comparing the temps from the two digitals (ET-732 and Taylor [not the pocket digital]), I stopped checking the smaller butt (the one with my version of the marinade) and just used the reading from the larger one as a proxy. The difference in weight between the two was just about a pound.
The guests were impressed. Either that or they were being diplomatic. Most remarked that the butt with my version of the marinade was tastier. Both were juicy, pulled easily. Thankfully, someone brought black eye peas and corn bread. One of the guests, whose wife is Filipino, brought a dish made with rice noodles, chicken, some vegetables. We also had cooked shrimp, pasta salad, black beans and rice, and baked beans.
All in all, it was a success. I share the spotlight with my grilling/barbecuing cyber-brethren. I went into this armed with some practical knowledge, and a game plan, and it worked.
Photos to follow in a few minutes in a separate post.
Thanks again, everyone, for your comments, suggestions and encouragement.