Preheat the oven to 350°F. Spray three 9-inch baking pans with nonstick baking spray.
In a large bowl, beat 2 sticks of butter, granulated sugar and 1 tablespoon vanilla at medium speed with a mixer until fluffy. Add the eggs, one at a time, beating well after each addition.
In a medium bowl, combine the flour, baking soda, cinnamon and salt. Add to the butter mixture alternately with the buttermilk, beginning and ending with the flour mixture, beating just until combined after each addition. Add the bananas and pineapple, beating until combined.
Using a measuring cup, spoon the batter into each of the three prepared pans, one scoop at a time to ensure an even distribution. Slam the pans against the counter to get any air bubbles out of the batter.
Bake until a wooden pick inserted in the center comes out clean, 25 to 35 minutes. Cool in the pans for 10 minutes. Remove from the pans and cool completely on wire racks.
Spread some Frosting between the layers. Spread the remaining frosting over the top and sides of the cake. Garnish with coconut and nuts if desired.
Frosting: Beat 2 sticks of butter and cream cheese together with a handheld electric mixer until smooth and creamy. Beat in the confectioners' sugar in increments. Lastly beat in the lemon juice and 1 teaspoon vanilla.