Martin Brinkmann writing on Ghacks has posted an interesting article “How to merge WordPress categories without plugins”.
“What you need to do is the following. Open Post > All Posts and filter the posts by the category that you want to merge with another. This displays all posts of the blog that have been assigned to that category.If you have lots of posts in that category, you may need to change the amount of posts displayed on a single page. Do that with a click on Screen Options at the top to change the post count per page.”
Read full article at: http://www.ghacks.net/2012/11/29/how-to-merge-wordpress-categories-without-plugins/