Line a rimmed 9x13-in. baking sheet with parchment paper. Microwave milk chocolate on high for 30 seconds in a small microwavable bowl. Stir. Continue microwaving, stopping to stir at 15-second intervals, until melted, about 30 seconds total. Spread melted chocolate onto left side of prepared baking sheet (about a 9x6-in. rectangle). Repeat melting process with bittersweet chocolate; spread onto right side of baking sheet. The two melted chocolates should meet in the middle of the baking sheet.
Sprinkle cranberries, white-chocolate chips, pomegranate arils, and cornflakes evenly over melted chocolate, gently pressing toppings into chocolate. Refrigerate until set, about 30 minutes.
Break bark into 16 pieces, about 2 to 5 in. each, and serve. Store in an airtight container in refrigerator up to 1 week.