Preheat oven to 350°. Spray the bottom only of a 13x9-inch baking dish with nonstick baking spray and flour.
Prepare and bake cake mix in the prepared dish according to package directions. Let the cake cool for 5 minutes.
1 15.25-ounce box yellow cake mix
In a medium bowl, stir together condensed milk, 1 cup of coconut milk, and 6 tablespoons of lime juice until blended.
1 14-ounce can sweetened condensed milk, 1 13.5-ounce can coconut milk, divided, 10 tablespoons bottled key lime juice, divided
Using the handle of a wooden spoon, poke holes in the warm cake, making sure the handle does not touch the bottom of the cake pan. Pour the coconut mixture over the warm cake.
Cover and refrigerate until chilled (approximately 4 hours).
In a large bowl that's been chilled, combine cream, confectioners’ sugar, remaining coconut milk, and the remaining 4 tablespoons of lime juice. Beat at high speed with an electric mixer and chilled beaters just until thickened. Spread over the cooled cake.