I posted this version for extending the downsized cake mixes quite some time ago. It is for one mix at a time. I don't use mixes very often, but I found that this makes a cake very close to homemade.
Eddie
I have used this modification when using a cake mix with excellent results.
To the average size cake mix add:
1/2 cup flour
1/2 cup sugar
1 tsp, baking powder
Substitute milk for the amount of water that the mix calls for
Use melted butter instead of oil, ( I used 1 cube )
1 tsp. vanilla
Add 1 extra egg
Mix as per the package directions. You will have a noticeably larger amount of batter. I used 2, 9" Wilton cake pans, which are a little deeper than most average 9" pans. If you are using regular 9" pans I would suggest using 3 pans, otherwise it may overflow. This recipe made the highest 9" cake I ever got out of a cake mix and the cake tasted almost like homemade. I haven't tried it with a chocolate cake mix, but I think I would use 1/4 cup cocoa and 1/4 cup flour instead of the 1/2 cup flour as listed above.
Eddie