Arrange squash halves cut side down on a baking sheet. Bake 30 minutes, or until tender. Dissolve the sugar in the melted butter. Brush squash with the butter mixture, and keep squash warm while preparing the stuffing.
Heat the olive oil in a skillet over medium heat. Stir in the garlic, celery, and carrots, and cook 5 minutes. Mix in the garbanzo beans and raisins. Season with cumin, salt, and pepper, and continue to cook and stir until vegetables are tender.
Pour the chicken broth into the skillet, and mix in the couscous. Cover skillet, and turn off heat. Allow couscous to absorb liquid for 5 minutes. Stuff squash halves with the skillet mixture to serve.