As long as the rack is similar I think you should be fine to run just one. if the strips of heat tape are all the same length they should all get to the same temp no matter how many shelves are on either side. what it would come down to is how well each rack holds that heat. if the tubs are the same size and racks are same material and thickness you should be fine. but if one is 3/8 pvc with open sides and the other is enclosed 3/4 plywood then you would need 2 thermostats.