Remove collard leaves from stems; discard stems. Massage leaves until fibers break down. Very thinly slice collard leaves to equal 8 cups. Place collards and cabbage in a large bowl. Toss with salt and 2 tablespoons of the sugar. Let stand 30 minutes, tossing occasionally. Squeeze liquid from collard mixture, and discard liquid.
Return collards to bowl. Add carrots, vinegar, oil, sesame seeds, celery seeds, black pepper, and remaining 1/2 tablespoon sugar. Toss well to coat.