It turns out that since the iCloud service release, users can not merge two Apple IDs into one.
However, you can still put all the content into a single iTunes library. Pick the library you want to use, and add all missing content to it. If your content includes any "protected" content from the iTunes Store, you will have to authorize to the account(s) from which the content was purchased.
By the way... If you have moved all the necessary files to another Apple ID account, you can deactivate the account that you no longer need. Head to privacy.apple.com.