Combine white sugar, brown sugar, salt, paprika, black pepper, garlic powder, onion powder, and cayenne pepper in a container with a lid; shake until rub is evenly combined.
Generously rub both sides of each rib rack with rub.
Preheat grill for medium heat and lightly oil the grate.
Place 1 rack of ribs, meaty-side down, in the center of main grill rack; cook for 5 minutes; transfer to the far side of top grill rack, meaty-side up. Repeat process with remaining rib racks.
Angle the rib racks on the top grill shelf so the meaty sides are facing out and they are stacked one in front of the other at an angle. Turn off heat directly below the ribs and turn other burner on low; close the grill lid and cook for 30 minutes.
Flip ribs so the closest side to the flame is the furthest side and the meaty side is still facing forward and the rack that was in the front is now in the back. Continue to cook, flipping every 30 minutes, until outsides are crispy and meat is tender, about 5 hours. Slice into small chunks.