Separate each graham cracker sheet into 4 crackers; place in a lightly greased 15- x 10-inch jelly-roll pan. Sprinkle chopped pecans over graham crackers.
Bring brown sugar and butter to a boil in a saucepan over medium-high heat. Boil 2 minutes. Pour brown sugar mixture evenly over graham crackers in pan.
Bake at 350° for 10 minutes. Quickly remove graham crackers to wax paper, using a spatula, and let cool completely.
Microwave semisweet chocolate morsels in a microwave-safe bowl at HIGH 30 seconds. Stir and microwave at HIGH 30 more seconds or until smooth. Drizzle chocolate evenly over cooled bars. Repeat procedure with white chocolate morsels.