Can articles be assigned to multiple categories or sections?
From Joomla! Documentation
This article is for Joomla! CMS Version(s) 
No, Articles and other content items cannot be assigned to multiple categories or sections. In Joomla! 1.0 and 1.5, content items are restricted to a single category in a single section. In Joomla! 1.6 and newer you are restricted to a single category in a category tree.
Possible work-arounds include:
- creating duplicate articles in multiple categories.
- using menus rather than dynamic lists of content items.
- using various third party extensions that simulate assignment to multiple categories.
- using key words to simulate categories and sections.