Preheat oven to 350°. Prepare and bake cake mix according to package directions, using a 13x9-in. baking pan. Cool on a wire rack.
Meanwhile, in a large bowl, whisk milks, 1/2 cup cream and rum. With a wooden skewer, poke holes in cake about 1/2 inches apart. Slowly pour milk mixture over cake, allowing mixture to absorb into cake. Let stand 30 minutes. Refrigerate, covered, 8 hours or overnight.
In a small bowl, beat remaining cream until it begins to thicken. Add confectioners' sugar; beat until stiff peaks form. Spread over top of cake. Sprinkle with coconut and pecans. If desired, drizzle servings with chocolate syrup.