Preheat grill pan or oven broiler (with the rack 3 inches from heat source) on high temperature.
Combine ingredients for marinade, and divide between two bowls (one bowl to marinate the raw meat and one bowl for cooking and serving).
Mix the beef, chicken, onion, tomatoes, and red pepper cubes in one bowl of the marinade and let sit. After 5 minutes, discard remaining marinade.
Place one piece of beef, chicken, tomato, onion, and red pepper on each of the 12 skewers.
Grill or broil on each of the four sides for 2–3 minutes or until completely cooked (to a minimum internal temperature of 145 °F for beef and 165 °F for chicken). Spoon most of the second half of the marinade over the kebabs while cooking.
Serve three skewers per serving. Drizzle the remaining marinade on top of each kebab before serving (use only the marinade that did not touch the raw meat or chicken).