I'm currently working on a Content Management System where authors can post articles. Before publishing, they must select a primary category for their article. For example "Entertainment", "Politics", or "Technology". This determines where to file the article in the overall hierarchy of the site. There are a predefined number of these to choose from, therefore, this is currently achieved using a radio button group.
However, if applicable, authors should be able to select multiple categories for an article, but with only one of them being the primary category. This is currently achieved listing the categories again, labeled 'secondary', and using checkboxes.
While the intentions are clear to the user, it makes the interface clunky, wastes space, is not scalable, etc.
Any ideas on a custom control that could combine these multi-selectors into one UI element?