I did some calculations and the amount of each ingredient, and therefore the total amount of batter produced, made for two 12x2 cakes will be slightly less than the amount required for a half sheet. It’s not short by much, but it will be less than the amount made if you were to follow the directions for a half sheet cake. If I were you I would just follow the directions a few pages later in TCB to make the half sheet recipe. This way you don’t have to worry about making any adjustments to the recipe. Good luck!