If you mean a 14x9x2 inch pan, then, yes, double the recipe, and it will take longer in the oven, since the pan and its contents is larger. You don't mention the temperature you use for the 30-minute cake, but you will likely be safe keeping at the original temperature, and adding 10 minutes to the baking time -- test for doneness, (toothpick in the middle) and add time if necessary in five-minute increments. That should do the trick! Good Luck.
