Preheat oven to 350 degrees and grease two bread pans. Toast the walnuts in a dry skillet over medium heat until fragrant, 3-5 minutes, then remove from heat and let cool. Place the butter and cream cheese into a large bowl, then mix until blended and creamy. Add the sugars and beat well to combine. Blend in the vanilla, then add the eggs, one at a time, beating after each one to blend. In a separate bowl, sift together the dry ingredients, then add to the wet and mix at a low speed, until just blended. Stir in the bananas and walnuts. Scrape dough evenly into well greased bread pans and bake for about 1 hour, or until a toothpick in the middle comes out clean. Cool bread on a wire rack for about 15 minutes, then remove from pans and allow to cool an additional 30 minutes before slicing, to avoid excessive crumbling.