Not sure about your thermometers but I refuse to use two therms in my rig. Why? Because one will make you second guess the other. Now of course you want to have some idea of what's going on with the temps in there. For me, I actually now prefer the turkey fryer therm. On the back of it is a nut na d you can turn that to calibrate it. I usually do the boiling water calibration but, some times I do the crushed ice in water calibration which is supposed to be more accurate.
For some reason I just tend to trust the dial therm more than the digital therms. However I am workign on buidling my own stoker which will rely on a digital therm. When that day comes I will only use that therm or I will be second guessing which one is right if I use two. That is if there is a difference between them.
In the end as long as the heat is low and the cook is slow I know what to look for when the meat is done.