In 2-quart saucepan, cook pasta in 4 cups boiling water 3 minutes. Drain; rinse with cold water to cool. Drain well. Meanwhile, cut thin slice from stem end of each bell pepper to remove top; reserve tops. Remove seeds and membranes; rinse peppers. To make peppers stand upright, cut thin slice off bottom of each.
In 10-inch nonstick skillet, cook turkey and pepperoni over medium-high heat 4 to 6 minutes, stirring occasionally, until turkey is no longer pink. Drain if necessary. Stir in pizza sauce, 1 cup of the cheese and the cooked pasta. Divide turkey filling among peppers.
Spray 5- to 6-quart slow cooker with cooking spray. Pour 1/2 cup water into cooker. Stand peppers upright in cooker; replace pepper tops. Cover; cook on Low heat setting 4 to 5 hours.
Remove pepper tops; reserve. Top filling with remaining 1/2 cup cheese. Cover; cook 5 to 10 minutes longer or until cheese is melted. To remove peppers from cooker, lift out with large slotted spoon, using tongs to support peppers (do not squeeze). Garnish with pepper tops.