i have not made the recipe mentioned above (scratch baker) but the website says that it will make 2 9" layers. A 9" circle has an area of 63.5 square inches. So 2 9" pans would have an area of 127.25. A 12 by 18 has an area of 216 square inches. If you double your chocolate cake recipe, it should fill 254.5 square inches (2x127.25) of pan to the same depth. Your 12 by 18 is less than that. So If you double the recipe, it will fill the 12 by 18 pan to a deeper level than it would have filled the 9" pans, or you'll have some batter left over for cupcakes. If you want to make 2 layers of 12 by 18, you will need four times the original recipe, but I would just mix 2 recipes at a time unless you have a really big mixer, and 2 pans.