Mix both boxes according to directions, and combine the batter. I generally use a large mix bowl, and add both boxes together, and add the other ingredients all at once. If you have only boxes of jiffy mix, use a brownie sized baking pan, 8 x 8, if you are going to use a box of regular cake mix, use a 9 x 13 inch pan. Bake at 350 degrees until done, this takes about 30 minutes, test for doneness.
Enjoy!