Preheat oven to 375°.
To prepare filling, cook nopalitos in boiling water 10 minutes. Drain.
Sprinkle the chicken with 1/2 teaspoon salt, cumin, and pepper. Heat a medium nonstick skillet coated with cooking spray over medium heat. Add chicken; cook 4 minutes on each side or until done. Remove chicken from pan; cool slightly, and shred with 2 forks.
Heat the pan coated with cooking spray over medium heat. Add onion and garlic; sauté 4 minutes or until tender. Stir in nopalitos, chicken, and 2/3 cup cheese. Cook 5 minutes or until thoroughly heated.
To prepare dough, combine masa harina, water, and 3/4 teaspoon salt in a medium bowl. Turn dough out onto a lightly floured surface; knead lightly. Press one-third of dough into an 11 x 7-inch baking dish coated with cooking spray. Spread filling over dough. Gently press remaining dough into a 4-inch square on heavy-duty plastic wrap. Cover with additional plastic wrap. Roll, still covered, into an 11 x 7-inch rectangle. Remove bottom sheet of plastic wrap; fit dough over filling. Remove top sheet of plastic wrap. Bake at 375° for 20 minutes. Sprinkle with 1/3 cup cheese. Bake an additional 5 minutes or until cheese melts.