Soak eight 8-inch skewers in water, at least 15 minutes. Preheat a grill or grill pan to medium high. Prepare the couscous as the label directs.
Meanwhile, toss the bell peppers, sausage, onion and tomatoes in a bowl with 1 tablespoon olive oil; season with salt and pepper. Thread onto the skewers, alternating the sausage and vegetables. Grill, turning, until the vegetables are slightly softened and the sausage begins to brown, 6 to 7 minutes.
Meanwhile, puree the parsley, cilantro and scallions in a blender with the remaining 2 tablespoons olive oil, the vinegar and 2 tablespoons water. Season with salt and pepper. Brush the skewers with some of the pesto and continue to cook, turning, until the tomatoes are tender and the sausage is charred, 6 to 7 more minutes.
Toss the couscous with half of the remaining pesto and season with salt and pepper. Serve with the skewers and the remaining pesto, for dipping.