I am making my first 11x15 cake. The pans says to use 11 cups of batter and a typical cake mix yeilds between 4-6 cups. I am using one of the CMD recipes. My question is on doubling the recipe. I thought this was a no-no, so do I mix one batch, put in in the pan, mix up another batch and then put that one on top? Will it still be OK to sit while I make up the other half? Do I have to refigerate it? Or is it OK to mix up 2 mixes together at once?
My 11 x 15 pans uses 2 cake mixes .. I just mix them both up and pour them into the pan and bake 
I always mix two batches at the same time for the 11x 15.
I did them separate once, but by the time I finished mixing the second, and the first had been sitting in the pan, the first had a slightly darker colour (I guess from sitting), so when I poured the second batch on top of the second, it looked kind of marbled!!
So I would definitely say mix them together.
Purple Petunia
I think i read in a box cake that you can mix two at a time but don't increase mixing time in think it was inside DH box
If you don't want to mix two at the same time, you could mix one ane pour in a large bowl and then mix the other and add to it and stir them together before pouring into the pan, I have done this before because I was afraid my KA would be to full to mix the batter up good.
I mix 2 batches together all the time, saves soo much time! I would not advise pouring your mix in the pan until it is ready for the oven. I have had several cakes sticking to the pan by waiting too long. 
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