You will most likely also have to edit your Agent.db file in your diablo folder with notepad and change supports multibox false to supports multibox true then save.
then show hidden folders in c drive and delete the agent folder from the battle.net folder. There is a post here that is more specific on that.
let me know if you need that as well and I will look it up and post the link here.
UPDATE: I did everything listed above BEFORE I did any diablo copying. If you havent done this already I suggest doing it FIRST. Let me know.