Stir together milk chocolate chips and peanut butter chips.
Coarsely chop 1-1/3 cups chip mixture in food processor or by hand; place in medium bowl.
Place remaining 2/3 cup chip mixture and shortening in small microwave-safe bowl.
Microwave at HIGH (100%) 45 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.
Pour melted chocolate mixture over chopped chips; stir to coat evenly.
With hands, form mixture into 1-inch balls.
Place in small candy cups, if desired.
Store in cool, dry place.