Heat oven to 250°F. Place popcorn, cashews and pecans in large roasting pan.
Combine brown sugar, butter and corn syrup in 2-quart saucepan. Cook over medium heat until mixture comes to a boil (7 to 8 minutes). Continue boiling 2 minutes. Remove from heat; stir in baking soda.
Pour butter mixture over popcorn mixture in pan; stir well. Bake for 60 minutes, stirring every 15 minutes. Remove from oven; place onto waxed paper or parchment paper. Cool completely. (DO NOT BREAK INTO PIECES.)
Place chocolate chips and 1 teaspoon shortening into resealable plastic food bag. Microwave on HIGH (100% power) for 30 to 45 seconds; knead bag. Continue microwaving at 15 second intervals, kneading until smooth. Cut tiny corner from bag; drizzle melted chocolate over popcorn mixture. Repeat with white baking chips and remaining 1/2 teaspoon shortening, microwaving on MEDIUM-HIGH (70% power). Drizzle over popcorn mixture. Let stand until chocolate is set (3 to 4 hours). Break popcorn mixture into pieces.
Store in container with tight-fitting lid or resealable plastic food bag.
Chocolate and white baking chips can be melted separately with shortening in a small bowl in microwave or in small saucepan over low heat until melted. Drizzle chocolate mixtures with fork or spoon.