Secondary flavoring of a stout using chocolate and coffee, how much to use?

For coffee, a half ounce to an ounce per gallon is a good place to start. Two ounces a gallon should definitely put you into "this just tastes like a cold cup of coffee" level. I feel like I've had lots of variance w/ how much flavor I get based on the beans I used or maybe based on variances in the grind.

For cherries--actually berries generally--I like to joke that if you don't balk at the price, you are probably not using enough. Doing a one gallon batch should make it a little easier on the wallet. A pound per gallon is probably around where I'd start for a stout. I normally freeze the fruit and stick it in a bag because racking off loose fruit is no fun at all. This won't work in jugs so maybe using some 100% cherry juice would be an easier option--but I can't offer advice as far as amounts.

I've never used cocoa nibs personally but I wanted to add that I think adding just a touch of vanilla makes beers taste a lot more chocolatey to me. For this purpose, I've always though vanilla extract works easily and tastes fine. Maybe start w/ about a tsp vanila per gallon. You kind of want to almost taste it.

I just brewed 5 gallons of coffee stout and added 3.5 oz of coarsely ground (lightly crushed in a plastic bag with a rolling pin) medium roast beans to the carboy after fermentation was complete. The coffee flavor was plenty strong, and I would not want any more. I didn't bother sanitizing the beans at all, but they were in dry-hopping tubes that I cleaned with StarSan before using.

The easiest way to do it is not add flavorings to the fermenter or kettle. Add them after the beer is fermented so you can test and control the amounts. After fermentation, before bottling or kegging, I pour 4 x 2oz. samples of the beer. I does each with a different, measured amount of flavoring then taste. I pick the one I like best then scale that amount of flavoring up to the size of the batch.

For how much coffee to use, check out the recent "Can You Brew It" where they tried to clone Terrapin's Wake-n-Bake stout. They worked from the exact recipe as given to them by the brewer at Terrapin. You can even buy the same blend of coffee they use commercially, if that interests you. I just listened to it this week because I also have a big stout sitting in the primary that I'm going to add coffee & chocolate to. Unfortunately, I don't remember exactly how much they used, because they added ground coffee to the secondary, like you plan to do. I'm going to do it differently.

My plan is to cold brew about a pint of sumatra mandheling using my French press. I'm going to make it a good bit stronger than if I were hot-brewing a cup for myself, probably twice as much coffee as I'd usually use. That would mean two heaping tablespoons (instead of one) per four ounces of water. I'll let it soak in pre-boiled, cooled water in the fridge for about 12 hours, then I'll press it off and pour it into the secondary before racking in the beer.

For my chocolate flavor, I plan to use cacao nibs. I got a bag of cracked ones as a gift. I want to sterilize them, so I plan to soak 8 ounces of them in some Maker's Mark overnight, then add the nibs and the bourbon to the secondary with the coffee before racking. I've never used nibs before, but I've read stuff from people who have and they were happy with the results. 8 ounces seems to be the sweet spot to get good-but-not-overwhelming chocolate flavor. The bourbon will add a bit of flavor, but it shouldn't be overpowering. Last night I did an experiment where I mixed 1/2 tsp of bourbon, 1/4 tsp instant espresso, and 1/4 tsp cocoa powder into 1 oz. of Fuller's London Porter, stirred it well to dissolve everything, then poured in the rest of the bottle. The bourbon was just barely there as a background note. The overall taste was pretty much what I'm looking for in my stout (except, you know, the base porter had less dark malt bitterness and the cocoa powder made it look like a glass of nesquik).