With two things like pizzas, that cook at the same temp, put them on different racks, and swap them around half way through the cooking time.
For things with a 50 degree difference like fries and chicken, the fries won't get crispy at the lower temperature. Assuming they cook for 20 minutes, start everything out at 400 for 10 minutes, then crank the oven up to 450. Take the chicken out 10 minutes later, and leave the fries in for another 5. If the cooking temp is 25 degrees different, then set the temp in between. I hope this helps.
For things with a 50 degree difference like fries and chicken, the fries won't get crispy at the lower temperature. Assuming they cook for 20 minutes, start everything out at 400 for 10 minutes, then crank the oven up to 450. Take the chicken out 10 minutes later, and leave the fries in for another 5. If the cooking temp is 25 degrees different, then set the temp in between. I hope this helps.