Assuming you install an app from the same source, I'm pretty sure the app itself (and not user data) is shared across all profiles. You should never have two instances of the same app (again, assuming the same source). Apps have app IDs to differentiate between them. App data is never shared across user profiles. Please correct me if I am wrong.
Now whether there is a difference between installing an app from your main profile into a secondary one vs downloading and installing it directly in the profile, I don't know. There shouldn't be.