Great question — answering as the UphillTech team:
When you put one item in the oven, the size affects the cooking time, because the thicker it is, the longer it takes for the heat to reach the middle. But if you put two things in, that's not a concern, unless they're so close together that they're basically "acting like" one big thing. According to this site , "The oven has a thermostat that keeps its temperature fairly constant, no matter what is placed in it (within reason). As long as the objects in the oven don't absorb so much heat that the oven can't keep up (or are so big as to prevent even circulation), it is unlikely that the number of things placed in the oven will affect the cooking time. It will take more heat to cook more mass...
Full answer by Josh: https://cooking.stackexchange.com/a/50108 (CC BY-SA 3.0).
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