I am nothing close to the pro that many are on this forum, but I do bake wedding cakes. To bake 2 12-inch layers, I have used the two middle racks before (I have a fairly small oven) and switched the layers (top<—>bottom) in the middle of the baking time as well as rotating the pans 180 degrees. That seems to work fine depending on how tall/high the cake is expected to rise—I have had the bottom layer hit the oven rack above and ruin the top of the bottom layer (fixable, but a mess). Your racks may be further apart than mine. If so, this wouldn’t be an issue with rose’s erecipes since most if not all are designed for 2” layers (and not 3”....that’s usually where I have a problem.
I usually prefer to bake only on a single rack in the center with at least 2 inches between the pans and each other, as well as between the pans and the sides of the oven. That works up to 9” in my oven. Any larger and I try to bake one pan per oven unless I am very pressed for time.
While the first large layer is baking, I usually leave the second layer of the tier on the counter (and not in the fridge) and haven’t had problems, but I would be very interested in what the others have to say about this!