1. Preheat oven to 400 degree F and butter two 8-inch square metal baking pans. Line bottom of pans with waxed paper; then butter paper.
2. Prepare corn muffin mix, 2 boxes at a time, adding eggs and milk, according to package directions; spoon each batch of batter into prepared pans to make corn bread. Smooth tops and bake in middle of oven until golden brown and a toothpick inserted in center comes out with a couple of crumbs clinging to it, about 20 minutes. Let corn bread cool in pans 10 minutes, then run a thin knife around edges and invert onto wire rack to cool completely. Cut into 3/4-inch cubes.
3. Preheat oven to 375 degree F. Lightly oil a large baking pan. Season cavity of squash with salt and pepper. Arrange squash, cut side down, on prepared pan and bake in middle of oven until almost tender, about 25 minutes. Remove pa from oven, carefully turn squash over with a spatula and let stand at room temperature while making stuffing.
4. Remove sausage from casing and cook in a large skillet over moderately high heat, stirring and breaking up sausage, until golden brown and no longer pink, about 5 minutes. Transfer with a slotted spoon to a bowl. Drain all but 2 tablespoons fat from skillet. Melt 2 tablespoons butter in skillet and add onions, celery, sage, and garlic; cook over moderate heat, stirring, until tender, about 10 minutes. Transfer to large bowl with sausage, add corn bread cubes and toss gently to combine. Season with salt and pepper.
5. Butter a shallow 3-quart glass baking dish. Spoon about 3/4 cup stuffing into each squash half in pan. Spoon remaining stuffing into prepared dish; drizzle with stock and dot with remaining 4 tablespoons butter. Bake stuffing and squash in upper and lower thirds of oven, uncovered, until top is crispy and golden brown, about 30 minutes. Makes 8 servings.