And what about Ivan the Terrible? (Although, both parts were made in different decades so I guess they probably should be broken up.)
They weren't made in different decades, they were made within two years of each other. Part 2 didn't get released anywhere for a long time because Stalin thought it was about him (which it was). Eisenstein had been dead for 10 years when it finally was shown (he died after making it, which is another reason why there's no Part 3).