Making two no-bake pies, double the recipe, or make the filling one at a time?

I would say that in that case, you can definitely scale the recipe without worrying.

Boil the apples with the other ingredients, and split the result once cooked to make two pies.

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You can usually scale up or down a recipe. The rough idea is simply to multiply the quantity of the ingredients of your recipe by a factor. Doubling means multiplying by 2, making half, by 0.5.

There is some exceptions to the above rule :

• When the recipe is dependent on the size of your cooking devices. For example, you can't scale a French omelette without scaling the size of the pan/your stove.
• Some delicate recipes might be difficult to scale, like a soufflé. In that case, it is because it can be hard to fold a big quantity of whipped white eggs in the thick batter in a homogeneous manner.
• Some ingredients are more tricky to scale : a recipe calling for 1 egg yolk might be difficult to scale down. Then, it is a matter of experience : if you are supposed to brush the egg over some pastry (to make it shiny and golden), then having 1 or ½ an egg does not make a big difference.

In general, savory recipe are easier to scale up or down. Just taste regularly what you are cooking, and adjust as you go. When it comes to pastry, this can be a bit more tricky, especially if you're planning to bake something delicate, but this is most of the time possible.

Final note: scaling is a lot easier when your recipe call for the ingredients in weight. I know this is not the case in some countries (USA, Sweden, to name a few), and to me, it makes scaling down those recipe a lot harder.