Gradually stir in milk; cook until thickened and bubbly, stirring constantly.
Stir in the remaining ingredients (horseradish, lemon juice, mustard, salt, crab, shrimp, cheeses, and cracker crumbs) and place in a buttered baking dish.
Garnish with pimento and parsley.
Bake 20-25 minutes.
Serves 8.
Suggested menu: wild rice, tossed salad, French bread, hot fruit compote, and stack cake.