I have two main front-end calendars. For the sake of simplicity here, one carries internal categories and the other external categories. People who can add internal category events need to have the option of adding them to an external category too, but the internal calendar shouldn't contain any external category events, because that a) defeats the purpose of having two separate calendars and b) clogs up the internal calendar with external events, which annoys users. I need a separate external calendar because external users need to be allowed to add events but not using an internal category.
So, how do I allow internal users to add events to both internal and external categories from just the internal calendar?