Preheat oven to 350°.
To prepare cake, coat 2 (8-inch) round cake pans with cooking spray. Dust pans evenly with 2 tablespoons flour.
Combine 1 cup granulated sugar, brown sugar, and oil in a large bowl; beat with a mixer at medium speed 2 minutes or until well blended. Add egg substitute and eggs; beat until well blended. Add pumpkin, beating until blended.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine flour, baking powder, and next 3 ingredients (through salt) in a medium bowl. Gradually add flour mixture to pumpkin mixture, beating just until blended. Spoon batter into prepared pans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes on a wire rack. Remove cake from pans; cool completely on wire rack.
To prepare frosting, beat butter and cream cheese at medium speed until creamy. Gradually add powdered sugar, beating until blended (do not overbeat). Add juice, stirring until blended.
Place 1 cake layer on a serving plate. Spread 1 cup frosting over layer, and top with remaining cake layer. Spread remaining frosting over top of cake. Sprinkle with pecans, and garnish with orange slices, if desired.
Pumpkin-pie spice is a combination of warm spices: cinnamon, ginger, nutmeg, allspice, cloves, and mace. Use it to enliven applesauce, vanilla ice cream, or tea.