I mix two at a time and i don't have any problems.
Whenever I use more than 1 cake mix, there seems to be less batter than if I had mixed them separately. I don't know why, but that always happens. I would mix separately.
I mix them together but 2 mixes will not give you 11 cups of batter. 1 mix makes 4.5 to 5 c batter. For my 11 x 15 pan I particularly like using a scratch recipe because I get enough batter without using a third mix. Of course, you can always use the extender recipe too.
I am unsure if you're talking about boxed cakes or scratch cakes. I have mixed up to 14 scratch cake mixes together, with no problems at all. This was for a cake for 500. It helps to have a commercial mixer and 2 ovens ; )
I am not sure why mixing two or more would be a problem. Special recipe maybe?? I have found that with sponge cakes, the ones where you have to beat the egg whites separately, if you bake two together it messes up, i think the extra weight doesnt it let it rise as much. But all this is guessing as im not a baker, let alone a food technologist : )
I have a 5qt ka and i mix 3 mixes togehter with no problems. You dont need to add anything just do as you normall would.
thanks all for the tips. I am using the Wilton sheet pan I think its 11x 15. I thought 2 mixes would be enough. If I use 3 I would have alot left over.
This is for the family not a paid order. I am using cookie cutters to make shapes out of cake then pouring quick pour fondant. What do I do with the left over batter?
I always mix up 3 mixes for my 11x15 cake pan...however, I normally have several orders so I use the extra batter to go in another cake. I mix up 4 cake mix boxes without a problem. I find that my mixes do not bake as well if I mix them up separately. Also...you could use your extra batter for some cupcakes and freeze them for another time. 
rydersmom...i'd like to also know about the meringue powder. what does it do?
I mix 2 mixes together with 1 extender recipe adn that works well for me.
rydersmom...i'd like to also know about the meringue powder. what does it do?
It is said to (some believe it, some don't) help your cake rise a little higher.
thanks all for the tips. I am using the Wilton sheet pan I think its 11x 15. I thought 2 mixes would be enough. If I use 3 I would have alot left over.
This is for the family not a paid order. I am using cookie cutters to make shapes out of cake then pouring quick pour fondant. What do I do with the left over batter?
With the left over batter you can make something like rum balls. bake the left over mix. let cool then crumble and mix with left over buttercream, ganache.
I mix two cake mixes at a time a lot and have never had a problem. Just add 1 tablespoon of meringue powder to the bowl.
What is the science behind this?
Thanks,
Sherri
I was told that it will firm up a cake batter. However, if too much is added (don't know how much is too much) it can also make a cake too dry!
I tried it once and didn't notice any improvement, so I figured why bother!!
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