I am assuming that your stove is electric. I utilize both only for preheating. Then I turn my selection knob to bake. By using both for baking, you are allowing the top of whatever you are making to get baked long before the rest of your food is ready. The top element is for broiling meats and preheating.
When a recipe calls for preheating the oven to a certain temperature, that is the only time you use both. Using both will not cook your food any faster. And it could be disastrous to your recipe. Specially when baking cakes. The top element is cooking the top of the cake before it even has had a chance to rise evenly. And it is a wasteful use of energy.