Dave provides a good thought. You could try to match up ribs by size to be closer in the finish, or just pull the ribs that are done, wrap in foil and hold in an ice chest. And do the same for the other racks as needed.
When I smoke ribs, I always wrap in foil and hold for about 2 hours prior to eating, this process makes them perfect for me. but you can always adjust the hold time as well to find your perfect spot.