By any measure, the Doctor Who Christmas Special was a tremendous success. In the UK, "Twice Upon A Time" was watched by 5.7 million people on Christmas Day, and it successfully introduced the world to Jodie Whittaker's 13th Doctor. Unfortunately, for all the episode triumphed in terms of the ratings, it also had real flaws. It was an unusual episode, both Peter Capaldi's swan song as the Doctor and a remarkable multi-Doctor story. As a result, it failed to make the most of two tremendous Doctors.
Why The Two Doctor Story Should Have Been Great (This Page)
A Multi-Doctor Arc Has A Very Simple Purpose
Multi-Doctor stories are traditional special events in Doctor Who history. The first, "The Three Doctors", aired in 1972 and 1973 to celebrate the 10-year anniversary of the show. A decade later, the show celebrated its 20-year anniversary with another multi-Doctor plot. Most recently, "The Day of the Doctor" was released as the largest ever simulcast of a TV drama, in honor of the 50th anniversary. Only 1985's "The Two Doctors" wasn't an anniversary episode.
The real draw for a multi-Doctor story is always the dynamic between the stars. Ever since 1972, the tradition has been for different incarnations of the Doctor to squabble and bicker; they'll point out one another's foibles and quirks, mock the size of one another's chins or shoes, and each will try to upstage the other. Given Doctor Who tends to reinvent itself with every regeneration, multi-Doctor episodes also allow fans to recognize how the show has changed. In the true spirit of Doctor Who, they bring past and present together.
The Two Doctors
Oddly enough, this is the second time another actor has portrayed William Hartnell's Doctor. Richard Hurndall played the role in "The Five Doctors", although in practice he didn't really take the time to act much like Hartnell. At the time, the BBC could be confident that few fans would have actually watched First Doctor episodes recently. There were no points of comparison.
Nowadays, classic Doctor Who is a big business and the DVDs top seller, so they needed to get an actor who could accurately portray Hartnell's Doctor. For "Twice Upon a Time", they chose David Bradley, who'd already played Hartnell already back in a 2013 drama exploring the early days of Doctor Who. As such, Bradley had studied the actor, with his take displaying many of Hartnell's small affectations. He really does create a sense of the First Doctor, and it's testament to Bradley's skill - after all, where most Doctors get to redefine the role around their own skills and style, Bradley is simply modeling his approach on Hartnell.
Further, the dynamic between Bradley and Capaldi is fantastic. Both are at the same point in their respective lives, battling against the need to regenerate and tempted to choose death rather than change. The First Doctor needs to learn what his future holds and to get a sense of hope. The Twelfth Doctor needs to be reminded why he does the things he does and to believe that he gives others hope. The symmetry is beautiful and poignant.