Cream together butter, 2 cups of sugar, and eggs. Add milk, flour, and baking soda. Put 1/2 of batter into greased loaf pans (1/4 in each pan). In a separate bowl, mix 2/3 cup sugar and cinnamon. Sprinkle 3/4 of cinnamon mixture on top of the 1/2 batter in each pan. Add remaining batter to pans; sprinkle with last of cinnamon topping. Swirl with a knife. Bake at 350° F for 45-50 minutes, or until toothpick tester come clean. Cool in pan for 20 minutes before removing from pan.