Place each butt on a large piece of aluminum foil. Squirt all sides of each with yellow mustard. Do not be shy with it.
Sprinkle all sides with the rub. Again, don't be shy with it.
Sprinkle a bit of brown sugar on all sides and rub in. This step is optional if you are using a sweet rub.
Pour a bit of Dr. Pepper into a spray bottle and lightly spritz all sides of the pork.
Tightly wrap the meat in the foil and place on large sheet pans in the fridge overnight.
Fire up your smoker or grill for cooking 225 F to 250 F. Add a bit of fruit wood once it is fully lit.
While waiting for the smoker to come up to temperature, inject each butt with more Dr. Pepper. Again, don't be shy with it.
Place pork onto the smoker fat-side up.
Mist the pork with additional Dr. Pepper every two hours. Cook 1 to 1 1/2 hours per pound or until the pork reaches 175 F when tested in multiple places with an instant-read thermometer.
Remove pork from the smoker and place into aluminum half pans. Add 1 cup of Cherry Dr. Pepper to each pan. Cover with foil and return to smoker until meat temperature reaches 205 F.
Drain any fat and excess liquid from the pans and uncover and cook a bit longer, until temperature returns to 205 F.
Remove butts and cover in foil and let rest 30-45 minutes.
Shred pork as desired and serve with the BBQ sauce on the side.