Cream 1/4 C. margarine in a medium bowl. Gradually add 1/2 C. brown sugar substitute, beating at medium speed of an electric mixer until mixture is light and fluffy. Add egg, and mix well. Combine 1-1/2 C. flour, baking powder, 1/2 tsp. ground allspice and salt. Add to creamed mixture alternately with skim milk, stirring just until moistened. Stir in finely chopped apple. Spoon batter into muffin pans coated with cooking spray, filling two-thirds full. Combine remaining 1/4 C. brown sugar substitute, 1/4 C. flour and 1/2 tsp. allspice in a small bowl. Cut in remaining 2 Tbls. margarine with a pastry blender until mixture resembles coarse meal. Sprinkle evenly over tops of muffins. Bake at 400 degrees F. for 20 minutes, or until lightly browned.