Using a large saucepan, cook ground beef and sausage over medium-high high until meat is browned. Combine meat, water, onions, bell peppers, tomato sauce, tomato paste, and garlic in a slow cooker. Cover; cook on high heat for 2-3 hours or on low heat for 5-6 hours. Spoon mixture onto buns; top with provolone cheese. You can put your sandwiches into the toaster oven or regular oven to melt cheese and toast buns. Serve!