E.T.
Good question! For me, yes - the disbelief went away, but I don't remember exactly when. Maybe by the time they were a year?
Talking to other parents of twins was helpful - I could share the excitement and also the crazy questions (what if one is smarter? why is one rolling over and the other isn't? why don't they like the same food? all those crazy twin questions!) There isn't a good Mothers of Multiple club near me, so I mostly relied on Twinstuff.com.