Why BOTH baking powder and baking soda in the same recipe?

To start with, I do understand there is a difference between baking powder and baking soda (mainly being you need to have a source of acid in the recipe in order for baking soda to work) and that’s not what I’m asking about. (Unless really I am?)

What I’m questioning is why some recipes will call for some of each? Rather than simply using a larger amount of one OR the other.

Is it like medicines, and actually work on different processes? For example, I take two different medicines to control my blood pressure. My doctor says they affect different processes in my body, and thus give a better result, and besides I’m taking the highest recommended amount of one of them already so she doesn’t want to up it.

Or, do b. soda and b. powder work on different time schedules, maybe? One starts the puffing up while the whatever is colder, and the other takes over when it gets hotter?

Or, maybe there is something similar to the ‘overdosing’ aspect of medicines? Like at higher concentrations either of them leaves the eater tasting something less pleasant, so a lower amount of each of them avoids that?

Anyone know what’s going on?

That actually is your answer. It’s about balancing the two with the acid already in the recipe. If you have enough baking soda to counteract the acid in the recipe, adding more won’t do anything. If you still need more lift, you need to add either baking powder, or more acid+baking soda, which make affect the taste and texture of the recipe.

ETA: Also baking power will react different than baking soda. Most of the time it’s double acting, so it will give a rise when you first mix and then again when it’s heated up.

Enalzi got it, but I’ll illustrate with an example :).

I make biscuits pretty often, and almost always make buttermilk biscuits. The base recipe I use for biscuits calls for regular milk and something like 2 tablespoons baking powder. The buttermilk variation calls for an additional 1/2 tsp of baking soda, presumably to counteract some of the acidity of the buttermilk.

I think baking powder is ph neutral (as in, it won’t change the acidity of your recipe), whereas baking soda is alkali. Use the former if your recipe doesn’t have enough acid to interact with baking soda to create CO2; use the latter if it does.

Aha! Okay, that makes sense – I was thinking more of a on/off effect, when the quantity of the acids and bases matters. (Horribly vague memories of HS chem class. Moles, or something, wasn’t it?)
Thanks to LHoD, too. I guess sometimes you WANT the tang of leftover acid to be there, and sometimes you don’t.