Baking brownies on both the top and bottom rack simultaneously and then switching their positions halfway through the baking time is not recommended. It can lead to uneven baking and may result in brownies that are either undercooked or overcooked.
When you bake multiple items in the oven at the same time, especially on different racks, the heat distribution can be inconsistent. The top rack may receive more direct heat, while the bottom rack might get less heat and more radiant heat. This can cause uneven baking and browning.
To ensure even baking and consistent results, it's best to bake brownies on a single rack at a time. If you want to bake multiple batches of brownies, it's better to do so one after another instead of baking them all at once on different racks.
If you have a large batch of brownie batter and want to bake it all at once, consider using a larger baking pan or dividing the batter into two pans and placing them on the same rack. This will help maintain even heat distribution and improve the chances of evenly baked brownies. Just be sure to adjust the baking time accordingly if you're using a larger or smaller pan than the recipe specifies.