Preheat the oven to 350 degrees F (175 degrees C).
Arrange squash halves cut-side down on a baking sheet. Bake until tender, about 30 minutes.
Dissolve sugar in melted butter. Brush squash with butter mixture; keep squash warm while preparing stuffing.
Heat olive oil in a skillet over medium heat. Stir in garlic, celery, and carrots; cook for 5 minutes. Mix in garbanzo beans and raisins. Season with cumin, salt, and pepper. Continue to cook and stir until vegetables are tender.
Pour chicken broth into the skillet, then mix in couscous. Cover the skillet and turn off heat. Allow couscous to absorb liquid for 5 minutes.
Stuff squash halves with the skillet mixture and serve.