P2 is also for probing food as a second reading only... you can't program against it. For instance, if you're doing a whole packer you might stick one probe in the point and the other in the flat so you can tell if both are progressing at similar rates and to tell if you're ready to open the smoker and do a physical check for done. The second probe can also just be a sanity check... I put one in each breast when doing turkey.
Yes, when your program finishes the set temperature will go to zero and the smoker will begin to cool. Some folks, instead, program the last step for a long time and a temp of 140 so it's more of a holding temperature instead of a cooking temperature.