Heat oven to 350°F.
Grease 15-1/2×10-1/2×1-inch jelly-roll pan.
Stir together brown sugar, butter and granulated sugar in large bowl; beat in eggs and vanilla.
Add flour and salt, beating just until blended.
Set aside 2 tablespoons each milk chocolate chips and peanut butter chips.
Stir remaining chips in batter; spread batter in prepared pan.
Bake 25 to 30 minutes or until wooden pick inserted in center comes out clean and surface is lightly browned.
Cool completely; cut into bars.
Place remaining 2 tablespoons milk chocolate chips, remaining 2 tablespoons peanut butter chips and shortening in small microwave-safe bowl.
Microwave at HIGH (100%) 30 seconds; stir. If necessary, microwave at HIGH an additional 15 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred.
Drizzle over bars. About