Spread a bit of icing on a cake stand or platter. Place an 8-in. cake layer on stand. Spread 1 1⁄2 cups icing on cake to edges. Top with second 8-in. cake. Spread a layer of icing on top and sides of cake, smoothing surface with a palette knife or spatula. Cut 4 straws to the height of the 8-in. cake. Poke straws into the centre of cake, about 3 in. apart. They should just reach the top of the cake and will help hold up the second tier. Place the 6-in. cake board on the centre of larger cake.