For the yogurt sauce: Stir together the yogurt, mint, lemon juice and cucumbers. Add salt to taste. Refrigerate while the meat marinates. (The sauce can be made up to 1 day ahead.)
For the kabobs: Mix the yogurt, cilantro, coriander, cumin, turmeric, allspice, 2 tablespoons of the olive oil, 1 1/2 teaspoons salt and 1/4 teaspoon pepper together in a large bowl. Add the beef and chicken (you can separate them into 2 bowls if you prefer) and toss to coat in the marinade. Cover and refrigerate for at least 1 hour and up to 3 hours.
Prepare a grill for cooking over medium-high heat.
Toss the peppers and onions with the remaining 1 tablespoon olive oil. Season with 1/2 teaspoon salt and pepper to taste.
Thread the chicken and beef onto separate skewers (about 5 pieces of meat per skewer), threading a piece of onion between each piece of meat--for even cooking, avoid overcrowding the skewers. Use 2 skewers to thread the peppers, to make them easier to turn. (Make 4 double skewers with 3 peppers each.)
Grill the kabobs, turning occasionally, until lightly charred on the outside and cooked through, 6 to 7 minutes for the beef (medium-rare), 10 to 12 minutes for the chicken, and about 12 minutes for the vegetables.
Serve the kabobs with lemon wedges and the yogurt sauce on the side.