The basic Jiffy cornbread mix requires 1 egg, and 3/4 cup of milk so if you are using 2 boxes then you would double those ingredients. The more ingredients you add the more box mixes you can use, making it denser. Depending on how many boxes you use, depends on the cook time. For 2 boxes we typically cook for about 20-25 minutes, or until the cheese is nice and melted.