In a large bowl, combine the back-fin crabmeat with the cream cheese, sour cream and 1 tablespoon of the chives. Spread a 1/2-inch-thick layer of the crab mixture across a large platter and top with the cocktail sauce. Scatter the lump crabmeat on top and garnish with the remaining 1 tablespoon of chives. Refrigerate the crab cocktail until slightly chilled. Serve with cucumber slices, crackers or toasts.