I need to make a 11 x 15 double layer cake. I use a doctored cake mix recipe. Does anyone know how many boxes of cake mix to use to make this cake? I'm thinking 2.???

Two, but you might have just a bit too much batter.

For a double layer cake, I would use four boxes of mix. I always use two for a one layer 11x15.

Yes, 4 boxes....2 for each layer. Just don't fill the pan more than 2/3 full!!

A box mix yields from 4-51/2 cups of batter.

An 11x15x2 sheet pan requires 11 cups of batter.

An 11x15x3 sheet pan requires 16 cups of batter.

http://www.wilton.com/cake/cakeprep/baking/times/index.cfm

The WASC cake recipe, using DH white yields a tad over 14 cups of batter.

Here's the expanded flavors recipe:

http://tinyurl.com/2cu8s4

Guess you could get by with 3 DH mixes using WASC cake recipe. (Or 1-1/2 doctored mixes per layer.)

HTH