Creamy Coleslaw: Combine sugar, pepper and celery seed. Stir in horseradish and mustard. Add mayonnaise. Mix until well blended. Stir in vinegar. Toss with cabbage, carrots and onions. Mix well to combine.
Sandwich Build: Place rye bread on flat surface. On bottom slice of rye bread, arrange 2 slices of corned beef. Top corned beef with 2 slices Swiss cheese. Place middle slice of rye bread on top of Swiss cheese. Spread ¼ cup Creamy Coleslaw on top of middle slice of rye bread. Top Creamy Coleslaw with 2 slices roast beef. Top roast beef with 2 slices turkey breast. Spread 1 tablespoon Thousand Island dressing on top portion of rye bread. Close sandwich with top slice of bread. Cut sandwich in half diagonally.