Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
In a medium bowl, mix together the pineapple, rhubarb, white sugar, brown sugar, gelatin, and marshmallows. Pour evenly in the bottom of the prepared pan. In the same bowl, prepare the cake mix according to package directions, substituting the reserved pineapple juice for the as much water in the recipe as you can. pour over the fruit, and spread evenly.
Bake for 1 hour in the preheated oven, or until a toothpick inserted into the center of the cake comes out clean. Invert the cake onto a serving platter while still warm, or cut into pieces, and turn upside-down when serving.