I have a cookie recipe I am trying for the first time, and it calls for both shortening and butter. I only have butter-flavored shortening on hand for the shortening. Should I reduce the amount of actual butter the recipe calls for? Or should I trek back to the store for regular shortening?
Butter and shortening behave differently in the cookie dough, so unless you are planning to do more adjustments, I recommend you stick to the given butter-to-shortening ratio. If a recipe uses both, it's often to get the best of two worlds.
This leaves the question of flavour.
If you use the butter-flavoured shortening, you will get a stronger "butter" flavour. This may or may not be what you want. So it's mostly a matter of taste (yours, to be exact) whether a trip to the store is in order or not. Without knowing more about your recipe, we can't even try an educated guess.