Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan. Prepare yellow cake mix according to package directions. Pour batter into the prepared pan and bake as directed until a skewer inserted into the center comes out clean. Let cool for 15 minutes.

While the cake is warm, use the handle of a wooden spoon to poke holes all over the top. In a small bowl, whisk together the sweetened condensed milk and the syrup from ONE can of peaches. Slowly pour this mixture evenly over the cake.

Drain the second can of peaches and dice them. Scatter the diced peaches over the cake. In a separate bowl, combine flour, brown sugar, granulated sugar, and cinnamon for the topping. Cut in the cold butter until the mixture resembles coarse crumbs. Sprinkle the crumble mixture over the peaches.

Set oven to a low broil. Broil the cake for 2-4 minutes, watching very carefully, until the topping is golden brown. Remove from oven.

Allow the cake to cool completely to room temperature. Once cooled, spread the thawed whipped topping over the surface. Refrigerate for at least 4 hours, or overnight, before serving.