Morrispe pretty much summarizes your options, though you can also look into Home Sharing and Family Sharing as ways to preserve some accessibility to multiple accounts and to DRM content in another account. You cannot merge, split, or join iTunes accounts as such.
Understanding Home Sharing - http://support.apple.com/kb/HT3819
iTunes: Setting up Home Sharing on your computer - http://support.apple.com/kb/HT4620
Troubleshooting Home Sharing - http://support.apple.com/kb/TS2972
Article discussing features of HS - http://www.macworld.com/article/1142729/homesharing.html
iTunes Home Sharing now works between users on same computer - https://discussions.apple.com/thread/3865597
Family Sharing - http://support.apple.com/kb/HT201060 - "Family Sharing makes it easy for up to six people in your family to share each other’s iTunes, iBooks, and App Store purchases without sharing accounts. Pay for family purchases with the same credit card and approve kids’ spending right from a parent’s device. And share photos, a family calendar, and more to help keep everyone connected."
Sharing purchased content with Family Sharing - http://support.apple.com/HT201085 - "With Family Sharing, family members get immediate access to each others’ music, movies, TV shows, books, and apps, and can download them to their own devices any time they like.*" "* Not all content and content types are available in all countries. All family members must use the same iTunes Store country or region. Music, movies, TV shows, and books can be downloaded on up to 10 devices per account, five of which can be computers. Apps can be downloaded to any devices the family member owns or controls.