Wilton makes an 11 x 15 that holds the equivalent of about two mixes. When frosted and decorated it would probably fit what you need. This is actually a go-to size for me. It is a great fit for larger house parties and bar-b-cues.
I have a fat daddios 11 x 15 that I use all the time. It is a very handy size. the area of 2 9" round cake pans is 127 sq inches. the area of an 11 x 15 rectangle is 165 sq inches. (this disregards the height, assuming that it will be the same for both)
for an 11 x 15 pan I make 1.5 times the recipe that I would use for 2 9" rounds. It will be just slightly taller. Or double your recipe and make cupcakes with the extra. HTH
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