When I was looking for a marble cake recipe, I was wary of just mixing together my chocolate and yellow cake batters, as all my cakes are scratch and the textures are pretty different. So would that not affect the combined cake?
Anyway, so I made the marble cake recipe from Rose's Heavenly Cakes, by Rose Levy Beranbaum. It was my dad's birthday cake and they said the cake was very good and lasted well. I got the book from the library, so if you don't have it, you can probably find it there. It was basically a sour cream yellow cake, then you divide the batter in two and add melted chocolate to half.
In my book, a choco marble cake is yellow batter with some chocolate batter dropped into it and swirled around.
You don't even have to make two different batters 
Just remove about 1/4 cup of batter (if it's a one recipe/mix size - more if larger) before pouring into the pan and add some UNsweetened cocoa powder to that, then drop spoonfulls into the yellow batter.
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