Adjustments when doubling a recipe
Question: When I am baking, can I just double or triple a recipe, or do I have to make other adjustments?
Answer: When you double or triple recipes for cakes, muffins, cookies and other baked goods, you should be mathematically precise when you double (or triple) the ingredients, but not the cooking times. When you double the batter, you might split it between two pans, or use a much larger pan: either way the cake(s) will take longer to bake than in the original recipe, but probably not twice as long.
My suggestion is this: test the cake at the time called for in your recipe. It won't be done, but check back in 5-minute increments until it is. And don't leave the cake in the oven with the door open when you test it, which will cause your oven temperature to drop. Take the cake out of the oven and immediately close the door while you test the cake.
P.S. And Cook'n will make the recipe adjustments automatically for you – just change the serving size!
Source: MARGE PERRY - myrecipes.com/how-to/cooking-questions