Prime rib is a high quality cut of meat, and it's very important that it's not overcooked. It's always your best bet to use a good remote meat thermometer to montitor the internal temperature of your prime rib as it smokes.
When smoking two beef rib roasts, you'd use the average time as if you were smoking just one of them. It will take a little longer than smoking just one, but not much.
Beef rib roast is the same cut as prime rib, but just a lower grade of beef. It's recommended to smoked it until its internal temperature is 5 to 10 degrees below the target temperature desired.
For even better flavor, remove the roast when it 15 degrees below the temperature you want. Let it rest for 15 minutes, covered with a tent of foil. Then place it in a very hot oven (450-500 degrees Fahrenheit) for 5 to 10 minutes, just long enough to give a brown crustiness to the surface.
