Prepare fajita mix according to package directions; add chicken and marinate as directed. In a large skillet, cook chicken in oil until no longer pink; set aside., In a large saucepan, saute onion in butter until tender. Stir in flour until blended. Gradually stir in broth. Bring to a boil. Cook and stir for 2 minutes or until thickened and bubbly. Add the tomatoes, process cheese and 1 cup Monterey Jack; cook and stir until cheese is melted., Stir in cream and reserved chicken; heat through (do not boil). Sprinkle with cheddar and remaining Monterey Jack cheese; top with guacamole and chips.