In a small saucepan, combine the chocolate chips and shortening. Heat over low or medium/low heat, stirring often until completely melted. Remove the pretzel sandwiches from the freezer and quickly dip each half-way into the melted chocolate. Return to the tray and repeat with remaining sandwiches. When all are dipped, return the tray to the freezer to set up completely. Store the sandwiches in the refrigerator until ready to serve.