You can use 2 boxes of cake mix in a 9x13 baking pan.
When you use 2 boxes of cake mix instead of one box of cake mix in the 9x13 baking pan it will make the cake extra thick and decadent.
One box of cake mix is standard for a 9x13 baking pan.
So when you use 2 boxes of cake mix for the 9x13 baking pan it also often requires baking the cake at a lower temperature of around 25F less than the normal baking temperature for the cake.
You also often need to increase the baking time for the 2 boxes of cake mix in the 9x13 baking pan by around 10 minutes to 15 minutes.
You can also use a toothpick check if the center of the cake is fully baked.
To prevent overflow of the cake mix when using 2 boxes of cake mix in the 9x13 baking pan, place a baking sheet on the rack below to catch any potential spillover and overflow of the cake mix.
And use a baking pan that is at least 2 inches deep to accommodate the extra volume of cake mix and cake as it bakes.
And because the cake batter will be much thicker when using 2 boxes of cake mix in the 9x13 baking pan, it can take significantly longer to bake through the middle of the cake
If you don't want a cake that is extremely thick, use the extra batter for some cupcakes.
The temperature that you bake 2 boxes of cake mix in a 9x13 baking pan is 325 F for around 45 minutes to 55 minutes.
Using the lower temperature to bake the 2 boxes of cake mix in the 9x13 pan prevents the edges of the cake from burning before the center of the cake is cooked, which is a result of the increased depth of the cake batter.
Use a metal or glass baking pan and use a toothpick to check for doneness.
If the toothpick comes out clean without any liquid or gooey cake batter then the cake is done.