Board Thread:The Panopticon/@comment-1293767-20151029072618/@comment-5918438-20161209230619

And that's exactly why we can't consider all continuous events one story. The Beast Below leads directly into Victory of the Daleks as well, but those are two separate stories.

If you look at the production side of things, The Christmas Invasion was directed by James Hawes, while The Parting of the Ways was directed by Joe Ahearne. (Not to mention they don't belong to the same run, either.) And don't forget you'd have to make those a three-parter to allow for the Children in Need Special.