Heat oven to 350°F. Grease bottoms only of 2 (8x4-inch) loaf pans or 1 (9x5-inch) loaf pan with shortening or cooking spray.
In large bowl, mix zucchini, carrots, sugar, oil and eggs with wire whisk. Stir in remaining bread ingredients. Divide batter evenly between pans.
Bake 8-inch loaves 1 hour to 1 hour 15 minutes, 9-inch loaf 1 hour 15 minutes to 1 hour 30 minutes, or until toothpick inserted in center comes out clean. Cool 10 minutes in pans on cooling rack.
Loosen sides of loaves from pans; remove from pans and place top sides up on cooling rack. Cool completely, about 1 hour.
In small bowl, beat spread ingredients with electric mixer on medium speed until smooth and fluffy. Serve with sliced bread. Cover; refrigerate any remaining spread. Wrap loaves tightly and store at room temperature up to 4 days, or refrigerate up to 10 days.