Special equipment: 15 wooden or bamboo skewers, soaked in water 30 minutes
In a small bowl, whisk together the olive oil, taco seasoning and lime juice.
Place the beef in one bowl, the chicken in another bowl and combine the peppers, tomatoes and onion in a third bowl. Divide the marinade among the three bowls and toss to coat.
Preheat a grill for cooking over medium-high heat. Thread the ingredients onto 15 soaked wooden or bamboo skewers, alternating pieces of beef or chicken with pepper, tomato and onion. (Keep the beef and chicken on separate skewers, as they have different cooking times.) Lightly oil the grill grates. Grill the kebabs, turning once, until cooked to desired doneness, about 6 minutes total for the beef kebabs and 8 to 10 minutes total for the chicken kebabs.
Serve with warmed soft tortillas, sour cream and fresh cilantro.