Combine cabbages in a medium bowl; toss well. Combine sugar, vinegar, water, oil, 1 teaspoon mustard, and pepper in a small saucepan; bring to a boil. Pour vinegar mixture over cabbage mixture, and toss gently. Cover and chill at least 2 hours.
Drain cabbage mixture, and set aside.
Combine mayonnaise and remaining 2 tablespoons mustard; stir well. Spread mayonnaise mixture evenly on 4 bread slices. Divide roast beef, Swiss cheese, and cabbage mixture evenly among prepared bread slices. Top with remaining 4 bread slices. Serve immediately.