Chocolate cake recipe requires both baking powder and baking soda?
As you have pointed out, baking powder is simply baking soda + acid. I've always, found recipes calling for both baking soda + baking powder a bit questionable. 

Since you only have baking powder, you should try to cut down the acidity coming from the other ingredients, most specifically cocoa. Try using Dutch Processed (neutralized) cocoa powder; as cocoa is naturally acidic. I assume the recipe is calling for white sugar, but if you plan to use brown sugar, you should also not do it as that's also quite acidic as well...

Taking these into consideration, I think you can then simply substitute baking soda with slightly more baking powder.