Line 13″x9″ baking pan with enough foil to overhang sides by 2″. In microwave-safe bowl, microwave 12 oz. semisweet chocolate in 30-sec. intervals, stirring well between each, until melted; stir in remaining chocolate until all chocolate is melted. Stir in ginger. Spread in pan. Let stand until set, about 1 hour.
In microwave-safe bowl, microwave 12 oz. white chocolate in 30-sec. intervals, stirring well between each, until melted; stir in remaining white chocolate until all chocolate is melted. Let stand until cool but still spreadable, 30-45 minutes. Spread over semisweet chocolate. Sprinkle with cranberries and pistachios; gently press fruit and nuts into chocolate.
Cover loosely; let stand at room temperature until set, at least 8 hours or overnight. Use foil to lift bark from pan; remove foil. Break into pieces.
