Not really fusion in the strict sense of the word, in the Teen Titans comic (the one base on the cartoon) Raven gets divide in many version of herself the titans then merge each version until raven is whole again.
http://funkyimg.com/u2/969/431/Teen_...042_-_p010.jpg
http://funkyimg.com/u2/825/217/Teen_...042_-_p011.jpg
http://funkyimg.com/u2/320/537/Teen_...042_-_p021.jpg