Microwave squash at HIGH 3 minutes. Cut each in half lengthwise. Carefully scoop out pulp into a bowl, leaving a 1/4-inch-thick shell. Finely chop pulp.
Meanwhile, devein shrimp, if desired. Coarsely chop shrimp.
Melt 1/2 cup butter in a large, deep skillet or Dutch oven over medium heat; add onion, and cook, stirring occasionally, 5 minutes or until tender. Reduce heat to low; add bell pepper and next 4 ingredients; sauté 12 minutes or until tender. Stir in squash pulp; cook, stirring often, 5 minutes. Stir in 1 cup breadcrumbs until combined. Remove from heat; stir in shrimp. Stir in salt and next 3 ingredients.
Microwave 2 Tbsp. butter at HIGH 10 to 15 seconds or until melted. Spoon squash mixture into shells, pressing down lightly. Sprinkle with remaining 1/4 cup breadcrumbs. Place squash in 2 (11- x 7-inch) baking dishes. Drizzle with melted butter.
Bake at 350° for 30 minutes or until lightly browned.
Note: For testing purposes only, we used an 1,100-watt microwave.