Preheat the oven to 200°F. Line a quarter-size (13- x 9- x 1-inch) rimmed baking sheet with parchment paper, making sure to let it extend 2 inches past the long sides.

Spread chopped semisweet chocolate in an even layer on the sheet. Sprinkle chopped white chocolate evenly over the semisweet chocolate. Turn the oven off and place the baking sheet inside. Bake the sheet until the chocolate melts, about 10 minutes.

Remove the baking sheet from the oven, and swirl the chocolate mixture with a wooden pick or the tip of the knife to create a marbled surface. Sprinkle peanuts and pretzels evenly over the melted chocolate, then stick it in the fridge to chill until it solidifies, about 30 minutes, or let it stand at room temperature until solid, about 1 hour.

Using the excess parchment as handles, remove the bark from the sheet and break it into bite-size pieces.