How long should I cook two 15 lb boneless prime ribs?

If you don't already have one, get a good instant read thermometer. ThermoWorks Thermo Pro is $34. Even better would be an oven-safe probe (or 2, one for each piece of meat) with a digital display that can sit on the counter top.

My recommendation, given the amount of meat, is to increase the initial sear time (at 500F) to 20-25 minutes or until you get enough exterior browning that you like. In other words, the meat should not be a pasty gray.

Then proceed at 325F. Take the temperature of each after 90 minutes. To see where the internal temperature is. You can estimate how fast the meat is cooking. Then take the temp at 30 minute intervals until you get to about 10degrees from your desired doneness. Here's where the probe thermometer can be handy: insert probe into center of the each piece at the beginning of the sear. You can see how fast the temperature is increasing and know the exact temperature to take out the roasts. No need to open the oven to take the temp.

I recently cooked (separately) 2 rib roasts using the ThermoWorks probe. My 5lb rib roasts took about 90 minutes to cook to medium rare. I took out the roasts when the internal temperature was 136F. During the resting period, carryover heat raised that internal temperature to 145F. It was perfect, especially the second one which my husband declared was the best he's ever eaten. I can't see myself making prime rib any other way now.

It's much more accurate to take the internal temperature than rely on length of cooking time. 30lbs of rib roast is too precious to be taking guesses.