It's starting to look incredibly like Doctor Who's 50th anniversary special The Day of the Doctor will have separate plot threads, for at least some of its running time, for David Tennant's and Matt Smith's Doctors.
Tennant, I think, will be galivanting about with Queen Elizabeth the 1st while Smith, likely in scenes intercut with Tennant's, will be in modern London and interacting with UNIT.
Then… timey wimey stuff will happen. Daleks. The Time War. John Hurt as let'snotargueaboutthatnow. Dimension-warping things in the National Gallery.
Call this all an educated guess. Maybe not educated enough. But it's certainly backed up by the new images from the special.
That's Ingrid Oliver as a character called, I believe, Osgood in that last picture. Why does she have a Fourth Doctor Scarf?
I expect our next clues as to what The Day of the Doctor is all about will come on Saturday when the first "proper" trailer for the show drops.