I make a lot of cake mix cakes. One cake mix, when baked, completely fills a 9x13 pan. Two mixes in one 9x13 would overflow while baking and cause a real mess in your oven.
Different mixes have different textures. For a light, fluffy cake, I use a cake mix that uses only egg whites, such as the Classic White mix.
A chocolate Devil's Food cake mix that uses whole eggs comes out comes slightly more dense, but still moist and fluffy.
It is very important to read and follow the directions for each mix. Precise measurements and not over-beating or under-beating matters. The baking time and temp will depend on what type pan you use (shiny metal, dark coated metal, glass, etc.) and what size pan you use. It is important to preheat the oven, too. All this info will be listed on the box.
Most oven temps vary slightly. I often use a 9x13 glass pan and bake at 330 instead of 350 degrees. Be sure your oven rack is in the center of your oven. And don't open the door while baking because a lot of heat escapes.