Heat gas or charcoal grill. Spray grill basket (grill "wok") with nonstick cooking spray. In medium bowl, combine chicken strips, bell pepper and onion stir-fry and 1/4 cup fajita sauce. Place chicken mixture in sprayed basket.
When grill is heated, place grill basket onto gas grill over medium heat or onto charcoal grill 4 to 6 inches from medium coals. Cover grill; cook 8 to 10 minutes or until thoroughly heated, shaking basket or stirring chicken mixture occasionally.
Meanwhile, place tortillas on ungreased cookie sheets. Sprinkle with 1 1/2 cups of the cheese.
Divide grilled chicken mixture evenly onto cheese-covered tortillas. Top each with 1 tablespoon fajita sauce and 1/4 cup remaining cheese.
With wide spatula, carefully slide pizzas onto grill. Cover grill; cook 3 to 6 minutes or until cheese is melted and crust is crisp, rearranging pizzas occasionally. Slide pizzas back onto cookie sheets. Cut into wedges.