Baking a couple of frozen pizzas at once may seem like a pretty easy task. After all, there are several racks in your oven, which seems perfect for several pizzas. If you've tried this in a traditional oven, you may have ended up with one scorched pizza and one underdone pizza. To prevent this, you need to understand your oven's heat zones. Photo by Ken Hawkins.
As the heat radiates from the element (usually found at the bottom of the oven) the air inside and the walls of the oven heat up as well. The hot air rises and circulates while the walls radiate heat from the sides. If you're just dealing with one pan, your best bet is to put it on the centre rack; it's the most even region. In the case of a pizza, one cooked on the bottom rack will end up with a burnt crust and underdone toppings, and one cooked on the top will end up with an underdone crust, but well-cooked toppings.
When you add a second pan, things get a little more complicated. Rick Martinez explains:
When you put two baking sheets in the oven at the same, in the bottom and top halves of the oven, you are creating three zones of heat. The first is an intensely hot zone at the bottom of the oven near the heat source. The second is a moderately hot zone at the top of the oven where the heat from the ceiling is trapped by the top sheet. And the third is a cooler zone in the middle of the oven in between the two baking sheets.
There are two ways around this. You could just cook one pizza at a time on the centre rack, or you could rotate the pizzas front to back and top to bottom halfway through, as suggested by Martinez. This will ensure a crispy, but not burnt, crust and evenly cooked toppings. (Of course, this tip isn't limited to pizze, but can be used any time you have two sheet pans in the oven at once, be they filled with pizzas, cookies, or dinner rolls.)
Never Burn Stuff in the Oven Ever Again [Bon Appetit]