Heat oven to 350°F.
Combine flour, walnuts, baking powder, baking soda and salt in bowl; set aside.
Combine sugar and butter in another bowl. Beat at medium speed, scraping bowl often, until well mixed. Add eggs and vanilla; continue beating until well mixed. Add flour mixture; beat at low speed until well mixed.
Remove half of dough from bowl. Add melted chocolate to remaining dough in bowl. Beat at low speed until well mixed.
Divide chocolate dough and white dough each into two equal portions. Roll each portion on lightly floured surface into 6-inch log. Place 1 chocolate roll on top of 1 vanilla roll; shape into 10x1 1/2-inch log. Repeat with remaining dough logs.
Place each log 3 inches apart onto ungreased cookie sheet. Bake 23-28 minutes or until set and lightly browned. Let cool on cookie sheet 15 minutes.
Reduce oven temperature to 300°F.
Cut logs diagonally into 1/2-inch slices with sharp serrated knife. Arrange slices on cookie sheet, cut-side down. Bake 14-18 minutes, turning once, or until golden brown and crisp. Place onto cooling rack. Cool completely.
Melt chocolate chips and 1 teaspoon shortening in 1-quart saucepan over low heat, stirring occasionally, until smooth. Repeat with white baking chips and remaining shortening. Drizzle over biscotti.