Most meals are made up of multiple dishes that have different ingredients and require different cooking times. This means that one oven temperature doesn't always fit the needs of every dish. This goes for the microwave, too. A little planning can help you come up with ways to cook your meals efficiently and still get everything hot at the same time.
You might want to step down the temperature to add a second dish after the first one's had a chance to heat up for a while. A lower than called for temperature during part of the cooking process can keep a companion dish from drying out too quickly while still allowing both dishes plenty of oven time. Whatever strategy you employ, evaluating the problem before you're ready to pop the pan in the oven is the key to a successful outcome.
You may want to reconsider preheating the oven before you use it, too. Many dishes can be started in a cold oven without a problem. You may not want to do this with bread, but casseroles and other simple dishes can take advantage of the building heat, which will save you time and money.