Preheat oven to 350 degrees.

Combine graham cracker crumbs, butter, sugars, salt, and cinnamon in bowl. Firmly press half of mixture into bottom and up sides of a deep, 9 ½-inch pie plate. Repeat with remaining mixture in second pie plate. Place on a rimmed baking sheet and bake until firm, 8 to 10 minutes.

Remove from oven, and sprinkle half of bittersweet chocolate over bottom of each crust. Return to oven to melt chocolate, about 1 minute. Spread chocolate in a thin layer on bottom and up sides. Let cool on a wire rack. Reduce oven temperature to 325 degrees.

In a large heatproof bowl set over a pot of simmering water, melt semisweet chocolate and butter, stirring until smooth. Remove from heat.

Mix pumpkin, milk, brown sugar, eggs, cornstarch, vanilla, salt, cinnamon, ginger, nutmeg, and a pinch of cloves in a medium bowl. Whisk ⅓ pumpkin mixture into chocolate mixture. Whisk in remaining pumpkin mixture until completely incorporated.

Pour half of pumpkin mixture into each crust. Bake until center is set but still a bit wobbly, 55 to 60 minutes. Let cool in pie dish on a wire rack. Refrigerate until well chilled, at least 8 hours (preferably overnight). Before serving, drizzle melted milk chocolate over top of both pies. Serve immediately.