re: Any suggestions for someone doing a Prime Rib for first time

You don't need to let them come to room temperature. That's a myth.

And yes, two smaller roasts will cook faster than one big one.

It will depend on what cooking temperature you are going low and slow with them as to how long it will take. The nice part about this method is you can let it rest for an hour and a half and then sear it to get it piping hot again.

So err on the side having it rest earlier than later.