Combine rhubarb, apples, 1/2 cup white sugar, and water together in a saucepan; bring to a boil. Cook and stir rhubarb mixture until rhubarb is tender and compote thickened, about 10 minutes.

Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.

Mix 2/3 cup white sugar, 1/4 cup margarine, egg, and vanilla extract together in a bowl; stir in milk. Whisk flour, baking powder, and salt together in a separate bowl; add to milk mixture until batter is just moistened, then pour into the prepared baking dish. Spoon enough compote over batter to create about a 1/2-inch-thick layer. Reserve any leftover compote for another use.

Stir oats, 1/2 cup margarine, 1/2 cup white sugar, and cinnamon together in a bowl until crumbly; sprinkle over compote layer.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 40 minutes.