Combine butter, rhubarb syrup and raspberry syrup; heat until butter melts and syrup thins. Add rhubarb; toss slightly; spread in 8x8x2-inch pan. Prepare cake mix using package directions; pour over rhubarb mixture.
Bake in 375° oven about 35 minutes or till done. Immediately run spatula around edges of pan and invert onto serving plate. Before lifting off pan, let syrup drain onto cake for 3 to 5 minutes. Cut in squares while warm. Top each serving with whipped cream.