Preheat the oven to 350 F. Lightly grease a 9x13-inch baking dish with non stick cooking spray.
In a mixing bowl, prepare the cake mix as directed with water, oil and eggs. Pour into the prepared baking dish.
Add the rhubarb and raspberries directly into the batter and press down lightly. Sprinkle the sugar over the top. Pour the cream over the top of the fuit and sugar.
Bake in the preheated oven for 50 to 60 minutes, or until toothpick comes out clean in the middle. If needed, loosely cover with foil for the last 10 minutes to prevent over-browning.
Let cool completely and chill for at least 4 hours, so that the custard layer on the bottom will set.
Serve Raspberry Rhubarb Cream Cake as is, or topped with vanilla ice cream or whipped cream.