Sorry for not seeing this yesterday. Cook time for cuts like Prime Rib (Ribeye roasts) aren't determined by weight, but rather, by thickness of the loin, chamber temp and desired finish IT. The only real impact you'll see on cooking 2 roasts at the same time is that you are putting more cold thermal mass into the smoker at one time, so it will take it longer to come back up to temp. Also, the smoker will have to work harder to maintain that chamber temp. On an electric or gasser, this isn't an issue really. On a stick burner or charcoal smoker, you'll burn through fuel at a faster rate.
One big thing you have to decide is how you want your PR to look. Do you want a consistent, edge to edge color ? Or, do you want a bullseye effect where the outer band of each slice is well done, with medium well meat next to it, then medium, yada, yada, yada. If you want the edge to edge, cook at a lower chamber temp, basically, 250 or less. If you want the bullseye effect, cook at higher temps, 300ish or more.
At 225ish, a good ball park figure is 3.5 to 4.5 hours to reach about 130, presuming the roast is coming right out of the fridge. Monitor the IT of the meat and at about 2.5 to 3 hour mark, if it seems to be going too fast for your timeline, drop the chamber temp. If it's going to slow, crank the temp up a bit.