Does acidity negate double-acting baking powder?

The short answer to your question is YES. The extra acid in the ingredients will hamper the second act of the double acting baking powder. The acids are timed/staged for reaction not the baking soda.

The Magic Baking Powder (happens to be in our kitchen, too) is mostly a single acting formula since monocalcium-phosphate is a low temperature acid (with apparently some double acting properties due to generation intermediate step of dicalcium phosphate; per your link). High temperature acid for second acts typically include sodium aluminium sulfate, sodium aluminum phosphate and sodium acid pyrophosphate.

You can try to counteract that by adding a bit of baking soda, but you run the risk of altering the taste and not having it all neuralized.

If you really want to get pedantic, use a pH meter to measure the acidity of your dough. I suspect tasting the dough might give an indication as well. (bitter alkaline, sour acidic)

Another test might be to mix your acidic ingredients in a bowl with some water and start adding measured baking soda until you see no more reaction (bubbles) and use that as a your basic of neutralizing your dough.

All said and done, I agree with SAJ14SAJ that you'll be just fine going with the existing recipe. There should be enough baking soda left to get something out of your double act. I also heard it from a world-class baker that most recipes can be done with only baking soda, let alone baking powder or double acting ones.

More technically, baking powder reacts a basic ingredient or alkali with an acidic ingredient. The reaction is enabled by the presence of water or heat. In a double acting baking powder, the 2nd reaction requires a certain amount of heat to be triggered, but it is still a reaction between an acid and a base.

So the limiting factor is whichever of the acidic or basic ingredient there is less of.

For example, if you added more lemon juice to a quick bread based on baking powder, there would just be a surplus of acid. If you add more baking soda to a balanced bread, there would be an excess of base (which would also taste a little metallic, as baking soda tends to).

However, there is no need to get the two ingredients perfectly balanced; that is just an incorrect supposition. Many or even most baked goods are probably a little bit acidic from fruit, buttermilk, or other ingredients, even after they are baked. If you look at a chart showing the pH of common ingredients, you will see that most are slightly acidic, such as flour and butter, even when you wouldn't expect it.

Update:

Per Harold McGee in On Food and Cooking, 2004 edition, pp. 535:

A rule of thumb for balancing baking soda and acid is 1/2 teaspoon of baking soda to:

• 1 cup of fermented milk
• 1 teaspoon lemon juice or vinegar
• 1 1/4 teaspoons cream of tartar

The first reaction is chemical in both.There are no more bubbles formed in baking powder vs baking soda but the corn starch combined with water in baking powder coats the bubbles if you will,by creating a less porous membrane allowing the gasses to expand more,therefore more body, when introduced to heat.