The BBC's trailer for the Doctor Who Centenary Special, "The Power of the Doctor," appears to tease two Doctors - with a shot showing two TARDISes. Doctor Who has a long tradition of celebratory multiple Doctor stories, often taking place at anniversaries. The upcoming Doctor Who Centenary Special celebrates the 100th anniversary of the BBC itself, and it would therefore be entirely appropriate for The Power of the Doctor - Jodie Whittaker's final episode as the enigmatic time traveler - to feature multiple incarnations of the Doctor.
Chris Chibnall's tenure as Doctor Who showrunner has been marked with a remarkable degree of secrecy, and it would be in character for him to launch a multi-Doctor story without announcing it. The BBC's recent trailer for The Power of the Doctor may have unwittingly hinted at it, however, in an explosive action shot featuring two TARDISes. It's a blink-and-you'll-miss-it moment, and the TARDISes aren't the focus of the shot, so many viewers have missed it. This certainly raises the possibility of another Doctor appearing in the Doctor Who Centenary Special.
Which Extra Doctor Could Appear In The BBC Centenary Special?
The most likely second Doctor is Jo Martin's Fugitive Doctor, introduced during the Chibnall era as a mysterious forgotten past incarnation of the Doctor. There have already been strong hints Martin will be reprising the role of the Fugitive Doctor in The Power of the Doctor, although most viewers assumed these would be in flashbacks as the Doctor discovered her past at last. The presence of the TARDIS seems to suggest multiple Doctors will be working together, however, so it's possible Martin's Fugitive Doctor and Whittaker's Thirteenth Doctor will be teaming up again.
The Second TARDIS Could Actually Be The Master
Alternatively, though, the second TARDIS could actually belong to the Master. Another shot in the trailer for the Doctor Who Centenary Special showed something very wrong with the TARDIS, with its traditional words replaced with mocking laughter. It's possible this is not the Doctor's TARDIS at all, but rather that it belongs to the Master, who has chosen to mimic the Doctor's familiar police box shape.
It's unclear how the Master survived the destruction of Gallifrey, but the most likely explanation is that he somehow escaped in a TARDIS of his own - one with a functioning police box. It would be perfectly in-character for the Master to disguise his vessel as the Doctor's TARDIS, perhaps even to trick the Doctor's allies into thinking he was an incarnation of the Doctor himself; the Master has always considered himself skilled at disguise and impersonation, and in tie-ins he's pretended to be the Doctor before. Alternatively, it's quite possible the Master has chosen to have his TARDIS take on this shape simply as a way of mocking the Doctor (which would fit with the words on the front of the police box). The Master hates the Timeless Child even more than he does the Doctor, because the very idea his regenerative abilities came from someone else is an insult to him. He'll surely want to find fresh and creative ways to torment his nemesis in the Doctor Who Centenary Special.