The recipe would remain the same as for the basic cake. Spread alternate layers of the two flavors. After this is done, gently swirl the batter through all the layers with a knife or skewer and then bake. Stick to whichever of the 3 methods given below works best for you.
2. One recipe batter:
You could divide one recipe batter into two and add different flavors to each portion for a vanilla and pistachio cake.
Sieve the flour and baking powder, do not add the cocoa powder here. Proceed with the rest of the recipe. In the end, divide the batter into 2 equal portions, add cocoa to one portion and mix well.
3. Doubling the recipe: Sieve the flour and baking powder, do not add the cocoa powder here. Proceed with the rest of the recipe. In the end, divide the batter into 2 equal portions, add cocoa to one portion and mix well for a chocolate and vanilla marble cake.