To prepare marinade, combine first 10 ingredients in a large bowl; set aside.
To prepare fajitas, trim fat from steak. Score a diamond pattern on both sides of the steak.
Combine 1 1/2 cups marinade, steak, and chicken in a large zip-top plastic bag. Seal and marinate in refrigerator 4 hours or overnight, turning occasionally.
Combine remaining marinade, bell peppers, and onion in a zip-top plastic bag. Seal and marinate in refrigerator for 4 hours or overnight, turning occasionally.
Prepare grill.
Remove steak and chicken from bag; discard marinade.
Remove vegetables from bag; reserve marinade.
Place reserved marinade in a small saucepan; set aside.
Place steak, chicken, and vegetables on grill rack coated with cooking spray; cook 8 minutes on each side or until desired degree of doneness.
Wrap tortillas tightly in foil; place tortilla packet on grill rack the last 2 minutes of grilling time. Bring reserved marinade to a boil.
Cut steak and chicken diagonally across the grain into thin slices.
Place the steak, chicken, and vegetables on a serving platter; drizzle with reserved marinade.
Arrange about 1 ounce steak, about 1 ounce chicken, 3 bell pepper wedges, and 1 onion wedge in a tortilla; top with 1 tablespoon salsa, about 1 teaspoon sour cream, and 1/2 tablespoon cilantro. Fold sides of tortilla over filling.