Baking two strata at once

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 that is placed in the oven, so the flame (or electric heating element) will be on more."

Don't forget that cooking times in recipes are always estimates, as some ovens hold temperature more tightly than than others; most aren't that accurate, and run hotter or colder than set; different sized ovens have different airflow; and food cooks differently at different altitudes. Always check the food when it's "supposed" to be done (or slightly before) and adjust accordingly!